merge/uma - Resolve configurator issues

[zvecr]

Description

Compiling: keyboards/merge/uma/uma.c                                                               ��[K’:
�plicit-function-declaration]
     oled_write(get_u8_str(get_current_wpm(), '0'), false);
��[K
cc1: all warnings being treated as errors

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).