haproxy: Add nossl variant

haproxy can be built without the huge libopenssl dependency to use it as
a simple http load balancer. This patch adds a haproxy-nossl variant to
the haproxy Makefile, which builds haproxy with the USE_OPENSSL flag
disabled.

Signed-off-by: Vikraman Choudhury <vikraman@gentoo.org>