Commit 7e462613 converted virtfs-proxy-helper to using libcap-ng. There
aren't any users of libcap anymore. No need to install libcap-dev.

Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Acked-by: Alex Bennée <alex.bennee@linaro.org>