Debian: we now need libpq-dev.
Debian: we now need libpq-dev.

psycopg 2.8 is now out, which means that the C library gets built from source,
so we now need libpq-dev when building.

Turns out the need for this package is already documented in
docs/postgres.rst.

matrix-org/synapse,matrix-org/synapse,matrix-org/synapse,matrix-org/synapse,matrix-org/synapse,matrix-org/synapse