Properly export ril_ex.h

Copying headers makes fixing compile errors hard, and breaks dependency
restrictions.  Move ril_ex.h to an exported include directory instead of
copying it.

Bug: 18903857
Change-Id: I4b59a01e442db0d3198def69a5a5ff17b2a41c66
diff --git a/libril/RilSocket.h b/libril/RilSocket.h
index 1539c11..619401a 100644
--- a/libril/RilSocket.h
+++ b/libril/RilSocket.h
@@ -16,7 +16,7 @@
 
 #ifndef RIL_SOCKET_H_INCLUDED
 #define RIL_SOCKET_H_INCLUDED
-#include "ril_ex.h"
+#include <libril/ril_ex.h>
 #include "rilSocketQueue.h"
 #include <ril_event.h>