s390/compat: fix compile error for !COMPAT

Fix this one for !COMPAT:

compat.h: In function ‘arch_compat_alloc_user_space’:
compat.h:292:2: error: implicit declaration of function ‘is_compat_task’

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