fix signal.h breakage from moving stack_t to arch-specific bits

in the previous changes, I missed the fact that both the prototype of
the sigaltstack function and the definition of ucontext_t depend on
stack_t.

include/signal.h

@@ -77,6 +77,8 @@ extern "C" {
 #define CLD_STOPPED 5
 #define CLD_CONTINUED 6
 
+typedef struct sigaltstack stack_t;
+
 union sigval {
 	int sival_int;
 	void *sival_ptr;