openconnect: avoid possible implicit dependency on liblz4

openconnect: avoid possible implicit dependency on liblz4

If the liblz4 library exists within the build environment, the openconnect
configure will pick it up and start depending on it, leading to the following
build error:

    Package openconnect is missing dependencies for the following libraries:
    liblz4.so.1

Disable LZ4 support in configure in order to avoid this implicit,
nondeterministic dependency.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>