poly2tri-c: move away from untrusted repo to one that is owned by its Nixpkgs maintainer

See also:
https://gitlab.gnome.org/GNOME/gegl/-/issues/214#note_1002908
https://github.com/NixOS/nixpkgs/pull/109812

pkgs/development/libraries/poly2tri-c/default.nix

···
 { lib, stdenv
-, fetchFromGitHub
+, fetchFromGitLab
 , autoreconfHook
 , pkg-config
 , glib
···
 
   outputs = [ "bin" "out" "dev" ];
 
-  src = fetchFromGitHub {
-    owner = "Mattey40";
-    repo = "poly2tri-c";
+  src = fetchFromGitLab {
+    domain = "gitlab.gnome.org";
+    owner = "jtojnar";
+    repo = pname;
     rev = "p2tc-${version}";
     sha256 = "158vm3wqfxs22b74kqc4prlvjny38qqm3kz5wrgasmx0qciwh0g8";
   };