Unregister pthread_atfork handlers on dlclose()

Bug: http://b/20339788
Change-Id: I874c87faa377645fa9e0752f4fc166d81fd9ef7e
diff --git a/libc/arch-arm64/bionic/crtbegin.c b/libc/arch-arm64/bionic/crtbegin.c
index fec0b11..7e2c5d7 100644
--- a/libc/arch-arm64/bionic/crtbegin.c
+++ b/libc/arch-arm64/bionic/crtbegin.c
@@ -67,3 +67,4 @@
 
 #include "../../arch-common/bionic/__dso_handle.h"
 #include "../../arch-common/bionic/atexit.h"
+#include "../../arch-common/bionic/pthread_atfork.h"