buildroot

qemu: add libssh2 optional dependency

Make sure that qemu uses libssh2 when libssh2 is enabled, for build
consistency.

Cc: Francois Perrad <francois.perrad@gadz.org>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
(cherry picked from commit d769377a41e172e1963351c168c97a1212561133)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>