libc: Add missing waitid() implementation.

Change-Id: I312ee608dbf9249e4886a10d45d13e3cda8a9042
diff --git a/libc/arch-arm/syscalls.mk b/libc/arch-arm/syscalls.mk
index f1d8fb1..c364d1c 100644
--- a/libc/arch-arm/syscalls.mk
+++ b/libc/arch-arm/syscalls.mk
@@ -3,7 +3,7 @@
 syscall_src += arch-arm/syscalls/_exit.S
 syscall_src += arch-arm/syscalls/_exit_thread.S
 syscall_src += arch-arm/syscalls/__fork.S
-syscall_src += arch-arm/syscalls/waitid.S
+syscall_src += arch-arm/syscalls/__waitid.S
 syscall_src += arch-arm/syscalls/__sys_clone.S
 syscall_src += arch-arm/syscalls/execve.S
 syscall_src += arch-arm/syscalls/__setuid.S