fontconfig: fixup fragile build after ab5fe171a

Sometimes it might fail due to timestamps suggesting some files needed
regenerating and failing to find gperf.  Now it should be OK, I hope.

pkgs/development/libraries/fontconfig/default.nix

@@ -34,9 +34,10 @@ stdenv.mkDerivation rec {
       sha256 = "0x2a4qx51j3gqcp1kp4lisdzmhrkw1zw0r851d82ksgjlc0vkbaz";
     })
   ];
-  # additionally required for the glibc-2.25 patch
+  # additionally required for the glibc-2.25 patch; avoid requiring gperf
   postPatch = ''
     sed s/CHAR_WIDTH/CHARWIDTH/g -i src/fcobjshash.{h,gperf}
+    touch src/*
   '';
 
   outputs = [ "bin" "dev" "lib" "out" ]; # $out contains all the config