android: set the HAVE_VISIBILITY define

... in order to limit the exported symbols only to the required ones.
Both compilers used with Android (GCC and LLVM) support this, so set it
unconditionally.

Cc: Chih-Wei Huang <cwhuang@linux.org.tw>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
diff --git a/Android.mk b/Android.mk
index 7038c3c..90cdcb3 100644
--- a/Android.mk
+++ b/Android.mk
@@ -39,6 +39,7 @@
 	$(LOCAL_PATH)/include/drm
 
 LOCAL_CFLAGS := \
+	-DHAVE_VISIBILITY=1 \
 	-DHAVE_LIBDRM_ATOMIC_PRIMITIVES=1
 
 include $(BUILD_SHARED_LIBRARY)