Install pkg-config

- pkg-config allows correctly getting the path of libraries; this is
  particularly useful on nix where the default paths *won't* be correct
diff --git a/modules/pkg-config.nix b/modules/pkg-config.nix
new file mode 100644
index 0000000..4123cbf
--- /dev/null
+++ b/modules/pkg-config.nix
@@ -0,0 +1,5 @@
+{pkgs, ...}: {
+  config.environment.systemPackages = [
+    pkgs.pkg-config
+  ];
+}