python3-dbusmock: only recommend python3-pygobject

dbusmock doesn't actually hard-depend on pygobject, some of the templates
use it but it isn't mandatory.

Change the RDEPENDS to RRECOMMENDS, and only recommend if g-i is enabled.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>