LICENSE: http_ext: fix GPL license version

LICENSE: http_ext: fix GPL license version

This is a followup of the previous commit: GH user @songliumeng initially
reported an issue with the GPL license version for event_hdl source file
which was fixed by the previous commit. It turns out the same mistake was
made in http_ext source file: due to a mixup between LGPL and GPL, GPL
version '2.1' was referenced instead of '2'.

Again, clarify that this is indeed GPL by making use of the banner
provided in doc/gpl.txt

This should be backported in 2.8 with b2bb925 ("MINOR: proxy/http_ext:
introduce proxy forwarded option")