Reduce ImageSpace::Init frame size

Fixes x86_64 build.

Change-Id: Ib636a1274e255a60bda47607570fedbe92a5e2cd
diff --git a/runtime/gc/space/image_space.cc b/runtime/gc/space/image_space.cc
index 1ebe5cc..6a61d78 100644
--- a/runtime/gc/space/image_space.cc
+++ b/runtime/gc/space/image_space.cc
@@ -1178,8 +1178,9 @@
       return nullptr;
     }
   }
-  ImageHeader temp_image_header;
-  ImageHeader* image_header = &temp_image_header;
+  // unique_ptr to reduce frame size.
+  std::unique_ptr<ImageHeader> temp_image_header(new ImageHeader);
+  ImageHeader* image_header = temp_image_header.get();
   {
     TimingLogger::ScopedTiming timing("ReadImageHeader", &logger);
     bool success = file->ReadFully(image_header, sizeof(*image_header));