Fix for gcc10 teensy_lc eeprom build warning

[zvecr]

Description

make clean handwired/onekey/teensy_lc:default
QMK Firmware 0.12.34
Deleting .build/ ... done.
Making handwired/onekey/teensy_lc with keymap default

arm-none-eabi-gcc (Arch Repository) 10.2.0
Copyright (C) 2020 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Compiling: keyboards/handwired/onekey/keymaps/default/keymap.c                                     Compiling: quantum/send_string.c                                                                   Compiling: keyboards/handwired/onekey/onekey.c                                                     Compiling: quantum/quantum.c                                                                       Compiling: quantum/bitwise.c                                                                       Compiling: quantum/keymap_common.c                                                                 Compiling: quantum/led.c                                                                           Compiling: quantum/keycode_config.c                                                                 [OK]
 [OK]
 [OK]
Compiling: quantum/matrix_common.c                                                                 Compiling: quantum/matrix.c                                                                         [OK]
Compiling: quantum/debounce/sym_defer_g.c                                                           [OK]
Compiling: quantum/mousekey.c                                                                      Compiling: tmk_core/common/chibios/eeprom_teensy.c                                                  [OK]
Compiling: quantum/process_keycode/process_space_cadet.c                                            [OK]
 [OK]
tmk_core/common/chibios/eeprom_teensy.c: In function 'eeprom_initialize':
tmk_core/common/chibios/eeprom_teensy.c:367:29: error: array subscript 0 is outside array bounds of 'uint32_t[1]' {aka 'long unsigned int[1]'} [-Werror=array-bounds]
  367 |     flashend = (uint32_t)(q - 1);
      |                          ~~~^~~~
tmk_core/common/chibios/eeprom_teensy.c:351:17: note: while referencing '__eeprom_workarea_end__'
  351 | extern uint32_t __eeprom_workarea_end__;
      |                 ^~~~~~~~~~~~~~~~~~~~~~~
Compiling: quantum/process_keycode/process_magic.c 

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Issues Fixed or Closed by This PR

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).