postgresql16Packages.wal2json: fix build on darwin

pkgs/servers/sql/postgresql/ext/wal2json.nix

···
   makeFlags = [ "USE_PGXS=1" ];
 
   installPhase = ''
-    install -D -t $out/lib *.so
+    install -D -t $out/lib *${postgresql.dlSuffix}
     install -D -t $out/share/postgresql/extension sql/*.sql
   '';