libnatpmp: make files in $out/lib executable

Fixes https://github.com/NixOS/nixpkgs/issues/139197

(cherry picked from commit be3bc77e3175f055e634ca1375d7cbdb60facfad)

pkgs/development/libraries/libnatpmp/default.nix

@@ -18,6 +18,10 @@ stdenv.mkDerivation rec {
 
   makeFlags = [ "INSTALLPREFIX=$(out)" ];
 
+  postFixup = ''
+    chmod +x $out/lib/*
+  '';
+
   meta = with lib; {
     homepage = "http://miniupnp.free.fr/libnatpmp.html";
     description = "NAT-PMP client";