Add accept4() syscall

Add accept4() using SYSCALLS.TXT and gensyscall

Change-Id: I6f19f29144186d15d46423e10f2cc4b4223719c6
diff --git a/libc/include/sys/socket.h b/libc/include/sys/socket.h
index 62a5300..0cddcd9 100644
--- a/libc/include/sys/socket.h
+++ b/libc/include/sys/socket.h
@@ -302,6 +302,7 @@
 __socketcall int shutdown(int, int);
 __socketcall int socket(int, int, int);
 __socketcall int socketpair(int, int, int, int*);
+__socketcall int accept4(int, struct sockaddr*, socklen_t*, int);
 
 extern ssize_t send(int, const void*, size_t, int);
 extern ssize_t recv(int, void*, size_t, int);