build: Build dmap plugin when soup 3 is used
Versions of libdmapsharing >= 3.9.11 (API 4.0) support libsoup 3.0. Building against libsoup 2.4 requires libdmapsharing's older 3.0 API, which grilo-plugins will now select when building against libsoup 2.4.
Signed-off-by: W. Michael Petullo mike@flyn.org