headers: end all headers with guard comment

Most headerfiles end with a /* <headerguard> */ comment, but it was
missing from some. The comment isn't the most important part of our
code documentation but consistency has an intrinsic value in itself.
This adds header guard comments to the files that were lacking it.

Closes #3158
Reviewed-by: Jay Satiro <raysatiro@yahoo.com>
Reviewed-by: Daniel Stenberg <daniel@haxx.se>