partially fix build.

Addresses the following compile time errors:

  In file included from frameworks/base/libs/hwui/LayerRenderer.cpp:24:0:
  frameworks/base/libs/hwui/RenderState.h:77:24: error: 'VirtualLightRefBase' has not been declared
       void postDecStrong(VirtualLightRefBase* object);
                        ^
  In file included from frameworks/base/libs/hwui/LayerCache.h:21:0,
                   from frameworks/base/libs/hwui/LayerRenderer.cpp:25:
  frameworks/base/libs/hwui/Layer.h:55:42: error: expected class-name before '{' token
   class Layer : public VirtualLightRefBase {
                                          ^
  In file included from frameworks/base/libs/hwui/RenderState.cpp:16:0:
  frameworks/base/libs/hwui/RenderState.h:77:24: error: 'VirtualLightRefBase' has not been declared
       void postDecStrong(VirtualLightRefBase* object);
                        ^

Change-Id: I2be439b373ea356b861f413ff6ce323e8d9baf2d