extensions#whitespace: fix false positive for check_mix_indent_file()
extensions#whitespace: fix false positive for check_mix_indent_file()

This patch prevents check_mix_indent_file() form notifying a "mix-ident-file"
when working where C/CPP using space and comment like:
/**
 * Some comment on the 1st column that shall not trigger check_indent_file()
 */
This kind of file can be found in linux kernel for example.

dalguji/vim-airline,vim-IDE/vim-airline,tschundler/dotfiles,rchicoli/vim-airline,wilywampa/vim-airline,yzhou61/vim-airline,tschundler/dotfiles,unphased/vim-airline,bling/vim-airline,HarmtH/vim-airline,tschundler/dotfiles,chrisbra/vim-airline