libosmocore: fix version in pkg-config file

All osmocon packets look for a file ".tarball-version",
when generating pkg-config files, which is absent in the git sources.
This results in an UNKNOWN version in the pkg-config, which confuses
configure of other osmocom packages, when checking the version.

pkgs/applications/misc/libosmocore/default.nix

@@ -22,6 +22,10 @@ stdenv.mkDerivation rec {
     hash = "sha256-Dkud3ZA9m/UVbPugbQztUJXFpkQYTWjK2mamxfto9JA=";
   };
 
+  postPatch = ''
+    echo "${version}" > .tarball-version
+  '';
+
   propagatedBuildInputs = [
     talloc
   ];