Support defaults for gensrcs modules

external/jemalloc/Android.bp has mulitple similar gensrcs, allow
using a defaults modules to simplify them.

Test: builds
Flag: NONE
Change-Id: If71a860c830e9502720caaf495b395ea28dd6983

genrule/genrule.go

@@ -754,6 +754,7 @@ func NewGenSrcs() *Module {
 func GenSrcsFactory() android.Module {
 	m := NewGenSrcs()
 	android.InitAndroidModule(m)
+	android.InitDefaultableModule(m)
 	return m
 }