staging/ion: fix build warnings

staging/ion: fix build warnings

This fixed below warnings.

include/video/ion_sprd.h:128:7: warning: ‘struct ion_handle’ declared inside parameter list
include/video/ion_sprd.h:128:7: warning: its scope is only this definition or declaration, which is probably not what you want
include/video/ion_sprd.h:129:29: warning: ‘struct ion_handle’ declared inside parameter list

drivers/staging/android/ion/ion.h:126:13: warning: ‘struct drm_gem_object’ declared inside parameter list
drivers/staging/android/ion/ion.h:126:13: warning: its scope is only this definition or declaration, which is probably not what you want

Change-Id: If51462fad12a1f7d20777835b39b1a276149cef1
Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>