Staging: android: add __user annotation in binder.c

This fixes the following sparse error
drivers/staging/android/binder.c:1795:36: error: incompatible types in comparison expression (different address spaces)

Change-Id: I3824cb700d0de0e24c94771b1441e639d7d4d18b
Signed-off-by: Bojan Prtvar <prtvar.b@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>