call into hwcomposer HAL when present

Change-Id: I70f31c69a9436a43860e78977442863ecba6d27b
diff --git a/services/surfaceflinger/Android.mk b/services/surfaceflinger/Android.mk
index a14bfb5..79772ed 100644
--- a/services/surfaceflinger/Android.mk
+++ b/services/surfaceflinger/Android.mk
@@ -5,6 +5,7 @@
     clz.cpp.arm \
     DisplayHardware/DisplayHardware.cpp \
     DisplayHardware/DisplayHardwareBase.cpp \
+    DisplayHardware/HWComposer.cpp \
     BlurFilter.cpp.arm \
     GLExtensions.cpp \
     Layer.cpp \