syscall.c: include <signal.h>

All users of NSIG macro must include <signal.h>.
This complements commit v4.10-184-g0e946ab.

* syscall.c: Include <signal.h>.
diff --git a/syscall.c b/syscall.c
index b0e20ee..8b5484e 100644
--- a/syscall.c
+++ b/syscall.c
@@ -34,6 +34,7 @@
 #include "defs.h"
 #include "native_defs.h"
 #include <sys/param.h>
+#include <signal.h>
 
 /* for struct iovec */
 #include <sys/uio.h>