Expose wait4 as wait4 rather than __wait4.

This helps strace(1) compile with one fewer hack.

Change-Id: I5296d0cfec5546709cda990abd705ad33d7c4626
diff --git a/libc/arch-arm/syscalls.mk b/libc/arch-arm/syscalls.mk
index 5cb46b5..9eb5136 100644
--- a/libc/arch-arm/syscalls.mk
+++ b/libc/arch-arm/syscalls.mk
@@ -4,6 +4,7 @@
 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/wait4.S
 syscall_src += arch-arm/syscalls/__sys_clone.S
 syscall_src += arch-arm/syscalls/execve.S
 syscall_src += arch-arm/syscalls/__setuid.S
@@ -180,7 +181,6 @@
 syscall_src += arch-arm/syscalls/ioprio_set.S
 syscall_src += arch-arm/syscalls/ioprio_get.S
 syscall_src += arch-arm/syscalls/uname.S
-syscall_src += arch-arm/syscalls/__wait4.S
 syscall_src += arch-arm/syscalls/umask.S
 syscall_src += arch-arm/syscalls/__reboot.S
 syscall_src += arch-arm/syscalls/__syslog.S