Build SF executable with USE_HWC2 when needed

The SurfaceFlinger executable wasn't picking up the USE_HWC2 flag when
enabled by the TARGET_USES_HWC2 BoardConfig, causing a mismatch in the
size of the SurfaceFlinger object. This fixes that by also passing
USE_HWC2 when building the executable.

Test: Cherry-pick from internal branch
Bug: 30302693
Change-Id: I1e41e5f548d4b8329590a082d9b69155ba365be9
diff --git a/services/surfaceflinger/Android.mk b/services/surfaceflinger/Android.mk
index 6a34898..08115ea 100644
--- a/services/surfaceflinger/Android.mk
+++ b/services/surfaceflinger/Android.mk
@@ -148,6 +148,10 @@
     LOCAL_CFLAGS += -DENABLE_CPUSETS
 endif
 
+ifeq ($(TARGET_USES_HWC2),true)
+    LOCAL_CFLAGS += -DUSE_HWC2
+endif
+
 LOCAL_SRC_FILES := \
     main_surfaceflinger.cpp