Explicitely disable liblz4 and external libtalloc support in order to avoid
implicit dependencies leading to the following error on build environments
that happen to provide liblz4 and libtalloc:
Package ocserv is missing dependencies for the following libraries:
liblz4.so.1
libtalloc.so.2
Signed-off-by: Jo-Philipp Wich <jo@mein.io>