Tell editors that these are Makefiles (or fragments thereof).

Change-Id: I0c5e29ac750b9605b1e8f823d6998d31a933b7a4
diff --git a/Docs.mk b/Docs.mk
index 13eddec..1f75a5e 100644
--- a/Docs.mk
+++ b/Docs.mk
@@ -1,3 +1,4 @@
+# -*- mode: makefile -*-
 # List of libcore directories to include in documentation.
 # Shared between libcore and frameworks/base.