scripts: gcc-wrapper: Add an allowed warning for alignment.c

 | kernel/arch/arm/mm/alignment.c:327:15: warning: 'offset.un' may be used
 uninitialized in this function [-Wuninitialized]
 | error, forbidden warning: alignment.c:327
 | make[2]: *** [arch/arm/mm/alignment.o] Error 1

Change-Id: Iadba4720a38997682036527a89b5adeb52ea6119
Signed-off-by: Sridhar Parasuram <sridhar@codeaurora.org>