debian: Add appropriate substitution variables to debian/control
Without this change, dh_gencontrol emits:
dpkg-gencontrol: warning: package python-notmuch: substitution variable ${python:Provides} unused, but is defined
dpkg-gencontrol: warning: package python-notmuch: substitution variable ${python:Versions} unused, but is defined
dpkg-gencontrol: warning: package notmuch-mutt: substitution variable ${perl:Depends} unused, but is defined
Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>