consolidate declarations of k_sigaction

Only alpha and sparc are unusual - they have ka_restorer in it.
And nobody needs that exposed to userland.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

arch/x86/include/asm/signal.h

@@ -46,11 +46,6 @@ struct sigaction {
 	sigset_t sa_mask;		/* mask last for extensibility */
 };
 
-struct k_sigaction {
-	struct sigaction sa;
-};
-
-#else /* __i386__ */
 #endif /* !__i386__ */
 #include <asm/sigcontext.h>