[PATCH] kbuild: add -Wundef to global CFLAGS

A recent change to the aic scsi driver removed two defines to detect
endianness. cpp handles undefined strings as 0. As a result, the test turned
into #if 0 == 0 and the wrong code was selected.
Adding -Wundef to global CFLAGS will catch such errors.

Signed-off-by: Olaf Hering <olh@suse.de>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>