libgui: enable tests in the 'tests' build.

This change makes the libgui makefile recurse into the tests directory
so that the tests get built by a top-level make when
TARGET_BUILD_VARIANT=tests.

Change-Id: I6f623cc4b86dfeb00b9d21823316dbd9def23110
diff --git a/libs/gui/Android.mk b/libs/gui/Android.mk
index 58bb0d3..b5737ff 100644
--- a/libs/gui/Android.mk
+++ b/libs/gui/Android.mk
@@ -38,3 +38,7 @@
 endif
 
 include $(BUILD_SHARED_LIBRARY)
+
+ifeq (,$(ONE_SHOT_MAKEFILE))
+include $(call first-makefiles-under,$(LOCAL_PATH))
+endif