linuxPackages.nvidia_x11.settings: use makeFlags in preBuild's libXNVCtrl build

pkgs/os-specific/linux/nvidia-x11/settings.nix

···
   preBuild = ''
     if [ -e src/libXNVCtrl/libXNVCtrl.a ]; then
       ( cd src/libXNVCtrl
-        make
+        make $makeFlags
       )
     fi
   '';