libc: Add missing waitid() implementation.

Change-Id: I312ee608dbf9249e4886a10d45d13e3cda8a9042
diff --git a/libc/arch-x86/syscalls/waitid.S b/libc/arch-x86/syscalls/__waitid.S
similarity index 90%
rename from libc/arch-x86/syscalls/waitid.S
rename to libc/arch-x86/syscalls/__waitid.S
index 9a5328b..4dd8c11 100644
--- a/libc/arch-x86/syscalls/waitid.S
+++ b/libc/arch-x86/syscalls/__waitid.S
@@ -2,11 +2,11 @@
 #include <sys/linux-syscalls.h>
 
     .text
-    .type waitid, @function
-    .globl waitid
+    .type __waitid, @function
+    .globl __waitid
     .align 4
 
-waitid:
+__waitid:
     pushl   %ebx
     pushl   %ecx
     pushl   %edx