Makefile.in: Point PIPEWIRE_CONFIG_DIR to source directory

Config files read from this variable - currently only media-session.d -
do not get copied to the build directory but are only ever copied during
install. In order to have media-session.d/with-pulseaudio detected when
`make run`'ing from the build directory PIPEWIRE_CONFIG_DIR must point
to the source directory containing these files.