qtcreator: unbreak build by disabling docs

Not ideal, but at least it builds now :-)

pkgs/top-level/all-packages.nix

···
   premake = premake4;
 
   qtcreator = qt5.callPackage ../development/qtcreator {
-    withDocumentation = true;
+    withDocumentation = false; # 'true' is currently broken with qt>=5.5
   };
 
   racerRust = callPackage ../development/tools/rust/racer { };