xtensa: pull signal definitions from signal-defs.h

This fixes the following build error in the current linux-next:

include/linux/signal.h:261:2: error: unknown type name '__sigrestore_t'
make[2]: *** [arch/xtensa/kernel/asm-offsets.s] Error 1
make[1]: *** [prepare0] Error 2
make: *** [sub-make] Error 2

that appeared after 32dae82 'consolidate kernel-side struct sigaction declarations'
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Chris Zankel <chris@zankel.net>

arch/xtensa/include/uapi/asm/signal.h

@@ -102,16 +102,7 @@ typedef struct {
 
 #ifndef __ASSEMBLY__
 
-#define SIG_BLOCK          0	/* for blocking signals */
-#define SIG_UNBLOCK        1	/* for unblocking signals */
-#define SIG_SETMASK        2	/* for setting the signal mask */
-
-/* Type of a signal handler.  */
-typedef void (*__sighandler_t)(int);
-
-#define SIG_DFL	((__sighandler_t)0)	/* default signal handling */
-#define SIG_IGN	((__sighandler_t)1)	/* ignore signal */
-#define SIG_ERR	((__sighandler_t)-1)	/* error return from signal */
+#include <asm-generic/signal-defs.h>
 
 #ifndef __KERNEL__