USB: Remove __KERNEL__ check from non-exported gadget.h.

Since the header file gadget.h isn't being exported to userspace,
there seems to be little point having a __KERNEL__ proprocessor check.

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>