init: remove last init.cpp global

By moving it into builtins.cpp..., but that's less bad than it is
now, especially since this is defunct in code targeting Q+.  Remove
the guards that init.h isn't being included by other files too as it's
not useful anymore.

Test: build
Change-Id: Ic564fcff9e8716ec924098b07a8c9d94ca25f960
diff --git a/init/builtins.h b/init/builtins.h
index 5db0d1c..7bbf6aa 100644
--- a/init/builtins.h
+++ b/init/builtins.h
@@ -40,6 +40,8 @@
     const Map& map() const override;
 };
 
+extern std::vector<std::string> late_import_paths;
+
 }  // namespace init
 }  // namespace android