Fix possible infinite loop when purging textures.

Change-Id: Ib05b398ae03e734da2dab0496df416fed4570b1c
diff --git a/libs/hwui/TextureCache.h b/libs/hwui/TextureCache.h
index b5e4c7c..847d69c 100644
--- a/libs/hwui/TextureCache.h
+++ b/libs/hwui/TextureCache.h
@@ -86,6 +86,8 @@
     uint32_t mSize;
     uint32_t mMaxSize;
     GLint mMaxTextureSize;
+
+    mutable Mutex mLock;
 }; // class TextureCache
 
 }; // namespace uirenderer