libc: Add missing waitid() implementation.

Change-Id: I312ee608dbf9249e4886a10d45d13e3cda8a9042
diff --git a/libc/SYSCALLS.TXT b/libc/SYSCALLS.TXT
index baac5d0..935639e 100644
--- a/libc/SYSCALLS.TXT
+++ b/libc/SYSCALLS.TXT
@@ -33,7 +33,7 @@
 void    _exit_thread:exit (int)	    1
 pid_t   __fork:fork (void)           2
 pid_t   _waitpid:waitpid (pid_t, int*, int, struct rusage*)   -1,7
-int     waitid(int, pid_t, struct siginfo_t*, int,void*)          280,284
+int     __waitid:waitid(int, pid_t, struct siginfo_t*, int,void*)          280,284
 
 # NOTE: this system call is never called directly, but we list it there
 #       to have __NR_clone properly defined.