define pid_t in wait.h, as the standard requires

Reference: /n/atom/patch/applied/waittypes
Date: Sun May 31 02:36:06 CES 2015
Signed-off-by: quanstro@quanstro.net

--- /sys/include/ape/sys/wait.h	Sun May 31 02:35:56 2015
+++ /sys/include/ape/sys/wait.h	Sun May 31 02:35:57 2015
@@ -2,6 +2,8 @@
 #define __WAIT_H
 #pragma lib "/$M/lib/ape/libap.a"
 
+#include <sys/types.h>
+
 /* flag bits for third argument of waitpid */
 #define WNOHANG		0x1
 #define WUNTRACED	0x2