guitone: fix qmake4Hook build pass guitone.pro which was removed in 9a8a4b6

pkgs/applications/version-management/guitone/default.nix

···
 
   buildInputs = [ qt4 qmake4Hook pkgconfig graphviz ];
 
+  qmakeFlags = [ "guitone.pro" ];
+
   meta = {
     description = "Qt4 based GUI for monotone";
     homepage = http://guitone.thomaskeller.biz;