package/libwebsockets: bump to version 4.3.1

This is quite a version jump, so there are a lot of individual changes,
see: https://github.com/warmcat/libwebsockets/blob/v4.3.1/changelog



LICENSE has also changed:

- More items with license "MIT" were added:
  - lib/misc/base64-decode.c
  - lib/plat/windows/windows-resolv.c
- One more item with a 2-Clause BSD license was added:
  - lib/misc/ieeehalfprecision.c
- Sublicense texts were copied into LICENSE

These should still be compatible with "MIT with exceptions" stated in
`libwebsockets.mk`.

Signed-off-by: Johannes Agricola <johannes.agricola@work-microwave.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>