SF: Define Hwc2::mock::Composer

This allows the unit test to use a mock in place of android::Hwc2::Composer.

Test: Code builds
Bug: None
Change-Id: Ia6f0df083d579cf69e97abc2ff7f2d3f5789a320
diff --git a/services/surfaceflinger/DisplayHardware/ComposerHal.h b/services/surfaceflinger/DisplayHardware/ComposerHal.h
index 4372362..77675fb 100644
--- a/services/surfaceflinger/DisplayHardware/ComposerHal.h
+++ b/services/surfaceflinger/DisplayHardware/ComposerHal.h
@@ -26,6 +26,7 @@
 #include <android/frameworks/vr/composer/1.0/IVrComposerClient.h>
 #include <android/hardware/graphics/composer/2.1/IComposer.h>
 #include <composer-command-buffer/2.1/ComposerCommandBuffer.h>
+#include <ui/GraphicBuffer.h>
 #include <utils/StrongPointer.h>
 
 namespace android {