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>

arch/s390/kernel/process.c

@@ -26,6 +26,7 @@
 #include <asm/io.h>
 #include <asm/processor.h>
 #include <asm/vtimer.h>
+#include <asm/exec.h>
 #include <asm/irq.h>
 #include <asm/nmi.h>
 #include <asm/smp.h>