consolidate kernel-side struct sigaction declarations

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/arch/s390/include/asm/signal.h b/arch/s390/include/asm/signal.h
index 8985359..d26e30e 100644
--- a/arch/s390/include/asm/signal.h
+++ b/arch/s390/include/asm/signal.h
@@ -28,11 +28,5 @@
         void (*sa_restorer)(void);
 };
 
-struct sigaction {
-        __sighandler_t sa_handler;
-        unsigned long sa_flags;
-        void (*sa_restorer)(void);
-        sigset_t sa_mask;               /* mask last for extensibility */
-};
-
+#define __ARCH_HAS_SA_RESTORER
 #endif