Fix mismatched field precision type compiler warning

Inserted cast to int so that third parameter matches expected
parameter precision type.

Original warning:

FileSystem.cc: In member function ‘Glib::ustring
GParted::FileSystem::mk_temp_dir(const Glib::ustring&,
GParted::OperationDetail&)’:
FileSystem.cc:81:69: warning: field precision should have type ‘int’,
but argument 3 has type ‘long unsigned int’