Fix the x86_64 vfork implementation.

Change-Id: I599507f5058e6196dc2d5b5653d06d5135dd8ac1
diff --git a/libc/arch-x86_64/x86_64.mk b/libc/arch-x86_64/x86_64.mk
index b17b138..212eebe 100644
--- a/libc/arch-x86_64/x86_64.mk
+++ b/libc/arch-x86_64/x86_64.mk
@@ -9,6 +9,7 @@
     arch-x86_64/bionic/__set_tls.c \
     arch-x86_64/bionic/sigsetjmp.S \
     arch-x86_64/bionic/syscall.S \
+    arch-x86_64/bionic/vfork.S \
     arch-x86_64/string/ffs.S \
     string/memcmp16.c \