vimfiler#util#get_vimfiler_winnr returns an appropriate winnr even if a buffer has multiple filetypes.

vimfiler can't close a window when the window has multiple filetypes. For example, `&filetype` is `project.vimfiler`. The root cause is that `vimfiler#util#get_vimfiler_winnr` can't return winnr if a buffer has multiple filetypes(because get_vimfiler_winnr compares filetypes using `==#` on closing a vimfiler window.). I think that comparing filetypes with regexp fixes this issue.
This function is used on closing a vimfiler window at https://github.com/Shougo/vimfiler.vim/blob/master/autoload/vimfiler/mappings.vim#L609.