syscall: add missing include statement to bootstrap error code and signal generator

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/54300054

src/pkg/syscall/mkerrors.sh

@@ -306,6 +306,7 @@ echo ')'
 (
 	/bin/echo "
 #include <stdio.h>
+#include <stdlib.h>
 #include <errno.h>
 #include <ctype.h>
 #include <string.h>