ART: Fix Mac build

Follow-up to 9821cceebf077cb747aed543582b3d740a03ea7f.

Bug: 28801010
Change-Id: I0bbdf83d64ea81e56824aa19bfae3baa341a2b73
diff --git a/runtime/oat_file.cc b/runtime/oat_file.cc
index 429cba8..7bec6e8 100644
--- a/runtime/oat_file.cc
+++ b/runtime/oat_file.cc
@@ -572,6 +572,7 @@
 
 void DlOpenOatFile::PreLoad() {
 #ifdef __APPLE__
+  UNUSED(shared_objects_before_);
   LOG(FATAL) << "Should not reach here.";
   UNREACHABLE();
 #else