xf86_input_wacom: also fix xorg-wacom.pc

I didn't notice that $(out) wouldn't get expanded there,
and that naturally caused issues for reverse dependencies.

pkgs/os-specific/linux/xf86-input-wacom/default.nix

@@ -46,9 +46,9 @@ stdenv.mkDerivation rec {
   ];
 
   configureFlags = [
-    "--with-xorg-module-dir=$(out)/lib/xorg/modules"
-    "--with-sdkdir=$(out)/include/xorg"
-    "--with-xorg-conf-dir=$(out)/share/X11/xorg.conf.d"
+    "--with-xorg-module-dir=${placeholder "out"}/lib/xorg/modules"
+    "--with-sdkdir=${placeholder "out"}/include/xorg"
+    "--with-xorg-conf-dir=${placeholder "out"}/share/X11/xorg.conf.d"
   ];
 
   meta = with lib; {