meson: Stop rebuilding modules over and over

meson: Stop rebuilding modules over and over

Instead of removing the sources every time and rebuilding, just use
rsync to preserve the timestamps and allow Make to do its job of
rebuilding if it changed.

There's a bug in meson that keeps building the testsuite even outside of
`ninja test`, but if the build result is cached, we can greatly minimize
the impact for developers.

Closes: https://github.com/kmod-project/kmod/issues/119
Signed-off-by: Lucas De Marchi <lucas.de.marchi@gmail.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://github.com/kmod-project/kmod/pull/121