Don't say Fnext_read_file_uses_dialog_p is const

Don't say Fnext_read_file_uses_dialog_p is const

It's const only if a windowing system is not used; don't say it's
const otherwise.  See:
http://lists.gnu.org/archive/html/emacs-devel/2015-01/msg00310.html
* lib-src/make-docfile.c (write_globals):
Add a special hack for Fnext_read_file_uses_dialog_p.
* src/fileio.c (next_read_file_uses_dialog_p): Remove.
Move guts back to ...
(Fnext_read_file_uses_dialog_p): ... here.
Don't declare as const, as make-docfile.c now has a special case
for this function.  This is an ugly hack, but it's better than
lying to the compiler.