requirements-swh: Add missing logging extra for swh.core

This fixes an import error during sphinx-apidoc processing.

requirements-swh.txt

 # Add here internal Software Heritage dependencies, one per line.
 swh.auth[django]
-swh.core[db,http]
+swh.core[db,http,logging]
 swh.counters
 swh.deposit[server]
 swh.fuse