databases/postgis21: Fix pkg-plist with RASTER option, DF support

PostGIS wouldn't build on DragonFly if the RASTER option was selected.
The fix is simple, set USES+= alias. While here, fix the check pkg-plist
failure seen when RASTER OPTION set. No revbump because RASTER is not
a default option, so no change to default package list.

Approved by: Just fix it