s390: add missing include to arch/s390/kernel/signal.c

This fixes the following warning:

CHECK   linux/arch/s390/kernel/signal.c
linux/arch/s390/kernel/signal.c:465:6: warning: symbol 'arch_do_signal_or_restart' was not declared. Should it be static?
Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/kernel/signal.c

@@ -16,6 +16,7 @@
 #include <linux/smp.h>
 #include <linux/kernel.h>
 #include <linux/signal.h>
+#include <linux/entry-common.h>
 #include <linux/errno.h>
 #include <linux/wait.h>
 #include <linux/ptrace.h>