ART: Move RegisterNatives helper

Move the helper closer to the users.

Test: m test-art-host
Change-Id: Id05ce5f60af54d0c5aef0c7d8932706c4602fc94
diff --git a/runtime/native/java_lang_StringFactory.cc b/runtime/native/java_lang_StringFactory.cc
index c1292ef..9c2e918 100644
--- a/runtime/native/java_lang_StringFactory.cc
+++ b/runtime/native/java_lang_StringFactory.cc
@@ -22,6 +22,7 @@
 #include "jni_internal.h"
 #include "mirror/object-inl.h"
 #include "mirror/string.h"
+#include "native_util.h"
 #include "scoped_fast_native_object_access-inl.h"
 #include "scoped_thread_state_change-inl.h"
 #include "ScopedLocalRef.h"