Expose wait4 as wait4 rather than __wait4.

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

Change-Id: I5296d0cfec5546709cda990abd705ad33d7c4626
diff --git a/libc/arch-mips/syscalls.mk b/libc/arch-mips/syscalls.mk
index 7e40c2e..0b8eccd 100644
--- a/libc/arch-mips/syscalls.mk
+++ b/libc/arch-mips/syscalls.mk
@@ -5,6 +5,7 @@
 syscall_src += arch-mips/syscalls/__fork.S
 syscall_src += arch-mips/syscalls/_waitpid.S
 syscall_src += arch-mips/syscalls/__waitid.S
+syscall_src += arch-mips/syscalls/wait4.S
 syscall_src += arch-mips/syscalls/__sys_clone.S
 syscall_src += arch-mips/syscalls/execve.S
 syscall_src += arch-mips/syscalls/__setuid.S
@@ -183,7 +184,6 @@
 syscall_src += arch-mips/syscalls/ioprio_set.S
 syscall_src += arch-mips/syscalls/ioprio_get.S
 syscall_src += arch-mips/syscalls/uname.S
-syscall_src += arch-mips/syscalls/__wait4.S
 syscall_src += arch-mips/syscalls/umask.S
 syscall_src += arch-mips/syscalls/__reboot.S
 syscall_src += arch-mips/syscalls/__syslog.S