Cygwin: fix signal.h with _POSIX_C_SOURCE=1

struct sigaction is POSIX.1-1990 but siginfo_t, which is used by its
sa_sigaction member, is POSIX.1b-1993.  Therefore it needs to be guarded
as well, and as part of a union, the struct size is protected.

Signed-off-by: Yaakov Selkowitz <yselkowi@redhat.com>

winsup/cygwin/include/cygwin/signal.h

@@ -325,7 +325,9 @@ struct sigaction
   __extension__ union
   {
     _sig_func_ptr sa_handler;  		/* SIG_DFL, SIG_IGN, or pointer to a function */
+#if __POSIX_VISIBLE >= 199309
     void  (*sa_sigaction) ( int, siginfo_t *, void * );
+#endif
   };
   sigset_t sa_mask;
   int sa_flags;