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
diff --git a/genrule/genrule.go b/genrule/genrule.go
index c0942d3..5b40768 100644
--- a/genrule/genrule.go
+++ b/genrule/genrule.go
@@ -754,6 +754,7 @@
 func GenSrcsFactory() android.Module {
 	m := NewGenSrcs()
 	android.InitAndroidModule(m)
+	android.InitDefaultableModule(m)
 	return m
 }