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 <[e-mail]>
Reviewed-by: David Gow <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

include/kunit/platform_device.h

···
 #ifndef _KUNIT_PLATFORM_DRIVER_H
 #define _KUNIT_PLATFORM_DRIVER_H
 
+struct completion;
 struct kunit;
 struct platform_device;
 struct platform_driver;