Support required property

This adds LOCAL_REQUIRED_MODULES to the generated Android.mk for any
dependencies in the "required" list of a module.

Change-Id: Ibcfe89dce7d247eb441af94e37388a59e71a75a9
diff --git a/android/module.go b/android/module.go
index f1af53a..bf9dfaa 100644
--- a/android/module.go
+++ b/android/module.go
@@ -120,6 +120,9 @@
 	// init.rc files to be installed if this module is installed
 	Init_rc []string
 
+	// names of other modules to install if this module is installed
+	Required []string
+
 	// Set by TargetMutator
 	CompileTarget Target `blueprint:"mutated"`