augeas: enable parallel building

Tested at -j4.

pkgs/tools/system/augeas/default.nix

···
   nativeBuildInputs = [ pkg-config ];
   buildInputs = [ readline libxml2 ];
 
+  enableParallelBuilding = true;
+
   meta = with lib; {
     description = "Configuration editing tool";
     license = licenses.lgpl21Only;