update droiddoc option to include api level 21 for javadoc tagging

Change-Id: Ifdb09267b0cdab4cd89aaea455de6270481ff230
diff --git a/Android.mk b/Android.mk
index b323650..7fa0e87 100644
--- a/Android.mk
+++ b/Android.mk
@@ -631,6 +631,7 @@
     -since $(SRC_API_DIR)/18.txt 18 \
     -since $(SRC_API_DIR)/19.txt 19 \
     -since $(SRC_API_DIR)/20.txt 20 \
+    -since $(SRC_API_DIR)/21.txt 21 \
 		-werror -hide 111 -hide 113 \
 		-overview $(LOCAL_PATH)/core/java/overview.html