s390/process: add missing header include

Add appropriate header file:

arch/s390/kernel/process.c:327:15: warning: symbol 'arch_align_stack' was not declared.

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