Add a dummy build recipe for generated RS cpp files.

Previously the RS cpp files are generated by the timestamp rule. Though
we have the generated RS cpp files depend on the timestamp file, we
don't have a build recipe. In such case gmake does some "optimization"
that it skip recompiling the generated cpp files, because it assumes the
generated cpp files are already up to date even if the rs files have
been updated.

Bug: 15313144
Change-Id: Ie69ecd2c788057d3619f9c7d2a125d44c4a534a1