docs: Add an explicit dependency on libcore/Docs.mk

We include libcore sources in apicheck / docs targets so we must
regenerate the stubs or api definitions whenever the list of libcore
sources changes.

Change-Id: I9383015069bc4064d5d06d3d540047cd408e24c7
diff --git a/Android.mk b/Android.mk
index e2eabed..c12f74a 100644
--- a/Android.mk
+++ b/Android.mk
@@ -731,7 +731,8 @@
 	$(foreach lib,$(FRAMEWORKS_SUPPORT_JAVA_LIBRARIES),$(call intermediates-dir-for,JAVA_LIBRARIES,$(lib)-res,,COMMON))
 
 framework_docs_LOCAL_ADDITIONAL_DEPENDENCIES := \
-    frameworks/base/docs/knowntags.txt
+    frameworks/base/docs/knowntags.txt \
+    libcore/Docs.mk
 
 samples_dir := development/samples/browseable