rpm: tell meson whether to use libssh or libssh2 explicitly

rpm: tell meson whether to use libssh or libssh2 explicitly

The %meson macro sets "--auto-features=enabled", thus any feature in the
RPM which has a "with_XXX" condition, needs to explicitly pass a
"-DXXX=state" arg to %meson to override the auto features setting.

The with_libssh and with_libssh2 conditions were not exposed to meson,
so if either was set disabled, then meson would fail the build if the
-devel packages were not found.

Reviewed-by: Pavel Hrdina <phrdina@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>