libxft: Fix reference to incorrect output

pkgs/servers/x11/xorg/overrides.nix

@@ -163,7 +163,7 @@ in
     preConfigure = setMalloc0ReturnsNullCrossCompiling;
     # the include files need ft2build.h, and Requires.private isn't enough for us
     postInstall = ''
-      sed "/^Requires:/s/$/, freetype2/" -i "$out/lib/pkgconfig/xft.pc"
+      sed "/^Requires:/s/$/, freetype2/" -i "$dev/lib/pkgconfig/xft.pc"
     '';
   };