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/m68k/include/asm/signal.h

@@ -30,9 +30,6 @@ struct sigaction {
 	sigset_t sa_mask;		/* mask last for extensibility */
 };
 
-struct k_sigaction {
-	struct sigaction sa;
-};
 #include <asm/sigcontext.h>
 
 #ifndef CONFIG_CPU_HAS_NO_BITFIELDS