Bug fix: Don't highlight C tags inside #if 0 (issue #57)

$VIMRUNTIME/syntax/c.vim is a bit of a mess, I can't make heads or tails
of it :-). In my simple #if 0 / #if 1 / #else tests this fixes the issue,
but I'm not sure that there are no corner cases left.

See issue #57 on GitHub:
  https://github.com/xolox/vim-easytags/issues/57