org-latex.el: fix bug when using #+INCLUDE before the first heading.
The LaTeX looks for strings to export before the first headline. Such
strings are defined as "before the first heading", so they normally never
match a heading regexp. However, #+INCLUDE can insert lines before the
first heading matching a heading regexp, causing these lines to appear
twice: as first lines and as normal headings.
The fix is to never include first lines when they match a heading regexp.
This was reported by Rasmus <rasmus.pank@gmail.com>.