ANDROID: GKI: USB: Gadget: add Android ABI padding to struct usb_gadget

Over time, this structure is being changed a bunch to fix issues found
in the gadget code as more and more people stress it.  So add padding to
handle any needed future changes that might occur.

Of course, now that the padding is there, there never will be any need
to change this structure again...

Bug: 151154716
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: I496797469268cc5d3cfafdb19c141b6b6196fe72