ART: Fixing build breakage.

Caused by Ic654a1fb9c1ae127dde59290bf36a23edb55ca8e.

Change-Id: Ia0c0fbcb6c8b141c5ee30a89ab6bc1c653916698
diff --git a/runtime/mirror/dex_cache.h b/runtime/mirror/dex_cache.h
index e04bde0..770c45d 100644
--- a/runtime/mirror/dex_cache.h
+++ b/runtime/mirror/dex_cache.h
@@ -35,7 +35,7 @@
 
 class String;
 
-struct StringDexCachePair {
+struct PACKED(8) StringDexCachePair {
   GcRoot<String> string_pointer;
   uint32_t string_index;
   // The array is initially [ {0,0}, {0,0}, {0,0} ... ]