postgresql16Packages.pg_relusage: fix build on darwin

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

···
   };
 
   installPhase = ''
-    install -D -t $out/lib *.so
+    install -D -t $out/lib *${postgresql.dlSuffix}
     install -D -t $out/share/postgresql/extension *.sql
     install -D -t $out/share/postgresql/extension *.control
   '';