[show] fix show version (sonic-net#1686)

#### 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`.

Signed-off-by: Longxiang Lyu <lolv@microsoft.com>

show/platform.py

@@ -23,7 +23,7 @@ def get_chassis_info():
     for k in required_keys:
         if chassis_info.get(k, '') in failed_vals:
             if platform_chassis is None:
-                import platform
+                import sonic_platform
                 platform_chassis = sonic_platform.platform.Platform().get_chassis()
             try:
                 chassis_info[k] = getattr(platform_chassis, "get_".format(k))()