test-hwc2: fix build error from ui/mat4.h include

ui/mat4.h is located at math/mat4.h on internal master. Change
test cases to reflect that.

Test: run the test-hwc2 tests

Change-Id: I1d2fb59efc2fcea40dc901664dc43841c5454353
diff --git a/services/surfaceflinger/tests/hwc2/Android.mk b/services/surfaceflinger/tests/hwc2/Android.mk
index b8c2133..203ced5 100644
--- a/services/surfaceflinger/tests/hwc2/Android.mk
+++ b/services/surfaceflinger/tests/hwc2/Android.mk
@@ -40,7 +40,8 @@
 LOCAL_STATIC_LIBRARIES := \
     libbase \
     libadf \
-    libadfhwc
+    libadfhwc \
+    libmath
 LOCAL_SRC_FILES := \
     Hwc2Test.cpp \
     Hwc2TestProperties.cpp \