kunit: platform: Resolve 'struct completion' warning

If the <kunit/platform_device.h> header is included in a test without
certain other headers, it produces compiler warnings like:

In file included from [...]
../include/kunit/platform_device.h:15:57: warning: ‘struct completion’
declared inside parameter list will not be visible outside of this
definition or declaration
   15 |                                                  struct completion *x);
      |                                                         ^~~~~~~~~~

Add a 'struct completion' forward declaration to resolve this.

Signed-off-by: Brian Norris <briannorris@chromium.org>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>