build: Don't require dbus development files

We don't actually need libdbus, only the dbus-daemon's installation
directory for system services, which in practice is always going to be
/usr/share/dbus-1/system-services.

Signed-off-by: Simon McVittie <smcv@debian.org>
2 jobs for dbus-services in 6 minutes and 57 seconds
Status Job ID Name Coverage
  Test
passed #10769502
build_autotools

00:06:55

passed #10769501
build_stable

00:02:38