adding a pkgconfig-depends on libpq

[GuillaumedeVolpiano]

fixes #49

[alexfmpe]

Nix CI server is currently down but I built locally without issue

$ git rev-parse HEAD && nix-build release.nix
971aea6d3e7f116424472975ab6dcd8415a1cf00
/nix/store/93q72vlbsyjls5ylgnm8l21y0qf98a1p-gargoyle-0.1.2.1
/nix/store/b1iqnjyr2wj6gp2q68zlw7bv41g9q78z-gargoyle-postgresql-0.2.0.3
/nix/store/3lib39ibmhc55k701pga3nki8acwjqsv-gargoyle-postgresql-connect-0.1.0.3
/nix/store/pnrr0xn2p8mx8qhdnznvd7ivgr6wh7pd-gargoyle-postgresql-nix-0.3.0.2
/nix/store/hfwl7vxnwnz2pyv8nv1clim59rppa39f-gargoyle-0.1.2.1
/nix/store/vrszdad1m2igz9xak2l2k6926hrsgqz6-gargoyle-postgresql-0.2.0.3
/nix/store/iysi6clfi5p5k8q8aj9p7nf8lrip41wp-gargoyle-postgresql-connect-0.1.0.3
/nix/store/xmsyb8nzkl2kywn1m1330hf9wggf8mm4-gargoyle-postgresql-nix-0.3.0.2
/nix/store/x47jia1qvzc7b1pnwcb7w6iwai6nj7k5-gargoyle-0.1.2.1
/nix/store/8zj3hk3hsh3rnp78l8nqnlbsnlzpcs4b-gargoyle-postgresql-0.2.0.3
/nix/store/k9215fbdqk51sfz5m7rb39bqv1k1l18b-gargoyle-postgresql-connect-0.1.0.3
/nix/store/rvz2bn7d6mb5v2nhw5r9cv5y2awi2n01-gargoyle-postgresql-nix-0.3.0.2
/nix/store/wfsk4lls4km59khlmrmvdkxssj548hj8-gargoyle-0.1.2.1
/nix/store/36xcz5afnig58j9gackh0s2l4lgkg1ha-gargoyle-postgresql-0.2.0.3
/nix/store/8m2fn3basyi34xdgbcn226r3ywq2fx1k-gargoyle-postgresql-connect-0.1.0.3
/nix/store/428xb80nrjk6sjw6wlln5zzj4h65b4i7-gargoyle-postgresql-nix-0.3.0.2
/nix/store/ic38r2f4nms9n19vqwwbw7pzq01s4hpr-gargoyle-0.1.2.1
/nix/store/jk45bfl8ry4k6ngd425jlm54is97ha4q-gargoyle-postgresql-0.2.0.3
/nix/store/3dkinshfi3vpr9hzxni96lhi63cl0qjr-gargoyle-postgresql-connect-0.1.0.3
/nix/store/3ng35nljdg5acar9zx2b702aiq99wfq6-gargoyle-postgresql-nix-0.3.0.2
/nix/store/frw0ags03qzv1z9v1cxyzrsdqgpfk386-gargoyle-0.1.2.1
/nix/store/7ia8gmq13wzxdwg6zfgyhhr5js4p6gra-gargoyle-postgresql-0.2.0.3
/nix/store/lqgk189kwrzi0sx13kwg77izp31c47c2-gargoyle-postgresql-connect-0.1.0.3
/nix/store/ncydl9rny6yd85km64b5ayrnlx42s62f-gargoyle-postgresql-nix-0.3.0.2

[alexfmpe]

@GuillaumedeVolpiano now on hackage: https://hackage.haskell.org/package/gargoyle-postgresql-nix-0.3.0.3

[GuillaumedeVolpiano]

Thanks !