Invalidate cached pid in vfork.

Bug: http://b/23008979
Change-Id: I1dd900ac988cdbe10aad3abc53240c5d352891d5
diff --git a/libc/arch-arm/arm.mk b/libc/arch-arm/arm.mk
index d72a160..b5283f6 100644
--- a/libc/arch-arm/arm.mk
+++ b/libc/arch-arm/arm.mk
@@ -42,6 +42,7 @@
     arch-arm/bionic/__restore.S \
     arch-arm/bionic/setjmp.S \
     arch-arm/bionic/syscall.S \
+    arch-arm/bionic/vfork.S \
 
 libc_arch_static_src_files_arm := arch-arm/bionic/exidx_static.c
 libc_arch_dynamic_src_files_arm := arch-arm/bionic/exidx_dynamic.c