Change xkb_map_new_from_fd to use FILE*

Change xkb_map_new_from_fd to use FILE*

i.e. xkb_map_new_from_file. The reason is that flex only works with
FILE's, so we must use fdopen on the file descriptor; but to avoid a
memory leak, we must also fclose() it, which, in turn, closes the file
descriptor itself.

Either way is not acceptable, so we can either:
* dup() the fd and use fdopen on that, or
* have the user call fdopen on his own, and accept a FILE* instead of an
  fd.

The second one seems better, and is standard C, so why not. We must add
stdio.h to xkbcommon.h though, which is regrettable, but not a big deal.

Signed-off-by: Ran Benita <ran234@gmail.com>