[show] fix show version

[lolyu]

What I did

Fix NameError from show version.

admin@str2-7050cx3-acs-02:~$ show version
Traceback (most recent call last):
  File "/usr/local/bin/show", line 8, in <module>
    sys.exit(cli())
  File "/usr/local/lib/python3.7/dist-packages/click/core.py", line 764, in __call__
    return self.main(*args, **kwargs)
  File "/usr/local/lib/python3.7/dist-packages/click/core.py", line 717, in main
    rv = self.invoke(ctx)
  File "/usr/local/lib/python3.7/dist-packages/click/core.py", line 1137, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/usr/local/lib/python3.7/dist-packages/click/core.py", line 956, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/usr/local/lib/python3.7/dist-packages/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
  File "/usr/local/lib/python3.7/dist-packages/show/main.py", line 960, in version
    chassis_info = platform.get_chassis_info()
  File "/usr/local/lib/python3.7/dist-packages/show/platform.py", line 27, in get_chassis_info
    platform_chassis = sonic_platform.platform.Platform().get_chassis()
NameError: name 'sonic_platform' is not defined

How I did it

Import sonic_platform before using sonic_platform.

How to verify it

admin@str2-7050cx3-acs-02:~$ show version

SONiC Software Version: SONiC.master.20303-a069cb0b1
Distribution: Debian 10.9
Kernel: 4.19.0-12-2-amd64
Build commit: a069cb0b1
Build date: Fri Jun 18 12:43:04 UTC 2021
Built by: AzDevOps@sonic-build-workers-000EBT

Platform: x86_64-arista_7050cx3_32s
HwSKU: Arista-7050CX3-32S-C32
ASIC: broadcom
ASIC Count: 1
Serial Number: JPE20424995
Model Number: DCS-7050CX3-32S-SSD
Hardware Revision: N/A
Uptime: 16:02:04 up  8:26,  2 users,  load average: 1.31, 1.34, 1.43

Previous command output (if the output of a command-line utility has changed)

New command output (if the output of a command-line utility has changed)