Implement system call wait w/ S.

src/userprog/syscall.c

@@ -3,6 +3,7 @@
 #include "threads/vaddr.h"
 #include "threads/interrupt.h"
 #include "threads/thread.h"
+#include "userprog/process.h"
 #include <stdio.h>
 #include <syscall-nr.h>
 
@@ -115,10 +116,9 @@ syscall_exec (const char *cmd_line UNUSED)
 }
 
 static int
-syscall_wait (pid_t pid UNUSED)
+syscall_wait (pid_t pid)
 {
-  //TODO
-  return 0;
+  return process_wait (pid);
 }
 
 static bool