need prototype to fix sigaction warnings

libc/sysdeps/linux/alpha/bits/kernel_sigaction.h

@@ -13,3 +13,6 @@ struct kernel_sigaction {
 	unsigned int sa_flags;
 	sigset_t sa_mask;
 };
+
+extern int __syscall_rt_sigaction (int, const struct kernel_sigaction *__unbounded,
+	struct kernel_sigaction *__unbounded, size_t);