s390: reenable gcc plugins for real

Martin's patch 6eedfaac ("s390: reenable gcc plugins") was lost in
the merge commit 85a0b791 ("Merge branch 'for-linus' of
git://git.kernel.org/pub/scm/linux/kernel/git/s390/linux"

) therefore
let's enable gcc plugins again.

Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>