bitbake.conf: require lz4 instead of lz4c

bitbake.conf: require lz4 instead of lz4c

With bitbake commit 'bb: compress: use lz4 instead of lz4c' we require
lz4, not lz4c [1].  Going as far back as Ubuntu 16.04/Fedora 25, they
are packaged together so it should be safe to simply adjust the
requirement from lz4c to lz4.

Note that the two are largely the same, but upstream has considered lz4c
deprecated since at least 2018 [2].

1. https://git.openembedded.org/bitbake/commit/?id=907472034b344e4eb73cfd43059a413469f52e1c
2. https://github.com/lz4/lz4/pull/553

Signed-off-by: Justin Bronder <jsbronder@cold-front.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>