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/Hwc2TestBuffer.cpp b/services/surfaceflinger/tests/hwc2/Hwc2TestBuffer.cpp
index a59f388..0b84184 100644
--- a/services/surfaceflinger/tests/hwc2/Hwc2TestBuffer.cpp
+++ b/services/surfaceflinger/tests/hwc2/Hwc2TestBuffer.cpp
@@ -24,7 +24,7 @@
 #include <gui/GraphicBufferAlloc.h>
 
 #include <ui/GraphicBuffer.h>
-#include <ui/vec4.h>
+#include <math/vec4.h>
 
 #include <GLES3/gl3.h>