meson: Enable oss module and padsp util

meson: Enable oss module and padsp util

Regarding the module:

This is unlike the autotools where liboss-util is built as a library,
here we build everything in the oss module, as apparently there's no
other consumer for liboss-util.

Regarding padsp:

Setting the install mode for padsp requires meson 0.47, so instead we
set padsp.in as executable in the git repository (which is what glib
does for gdbus-codegen btw).

Signed-off-by: Arnaud Rebillout <arnaud.rebillout@collabora.com>