gallium: build pipe-loader for both rusticl and clover

What does this MR do and why?

This MR includes the pipe-loader target when rusticl is being built, which was previously only a dependency of clover. Previously, attempting to build mesa with rusticl and without clover failed at the installation step, complaining that lib/gallium-pipe did not exist.

Builds and runs clinfo properly, with rusticl and both with and without clover.