build: fix pgsql fall-back configuration of CFLAGS

build: fix pgsql fall-back configuration of CFLAGS

When using mysql_config and pcap_config to configure `CFLAGS`, one
requests the actual flags:

  $mysql_config --cflags
  $pcap_config --cflags

By constrast, when using pg_config, one requests the include-directory:

  $pg_config --includedir

Therefore, the `-I` option has to be explicitly added.

Fixes: 20727ab8b9fc ("build: use pkg-config or pg_config for libpq")
Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>