spec: Add runtime requirement for libssh2

libssh2 unfortunately doesn't support symbol versioning so RPM can't
figure out what version is needed for the currently installed libvirt
package. This patch adds a runtime requirement, so that the correct
version of libssh2 can be installed along with libvirt.
(cherry picked from commit cb4f41b8)

libvirt.spec.in

@@ -1002,6 +1002,9 @@ Requires: cyrus-sasl
 # work correctly & doesn't have onerous dependencies
 Requires: cyrus-sasl-md5
 %endif
+%if %{with_libssh2_transport}
+Requires: libssh2 >= 1.3.0
+%endif
 
 %description client
 Shared libraries and client binaries needed to access to the