Make DRM libraries optional

Change-Id: I0b591b75a035bc6d8049c85b595966845fedaa15
diff --git a/drm/drmserver/Android.mk b/drm/drmserver/Android.mk
index 46a51bc..5df2ff8 100644
--- a/drm/drmserver/Android.mk
+++ b/drm/drmserver/Android.mk
@@ -41,4 +41,6 @@
 
 LOCAL_MODULE:= drmserver
 
+LOCAL_MODULE_TAGS := optional
+
 include $(BUILD_EXECUTABLE)