add api level 13 flag for docs

Change-Id: I03a7739c923b1f38de6b4e77374daed438559548
diff --git a/Android.mk b/Android.mk
index 21307eb..187731d 100644
--- a/Android.mk
+++ b/Android.mk
@@ -372,6 +372,7 @@
     -since ./frameworks/base/api/10.xml 10 \
     -since ./frameworks/base/api/11.xml 11 \
     -since ./frameworks/base/api/12.xml 12 \
+    -since ./frameworks/base/api/13.xml 13 \
 		-werror -hide 113 \
 		-overview $(LOCAL_PATH)/core/java/overview.html