[github] Add organization to sponsorship links

There is no viable way to link to the list of sponsorship recipients.
Add the organization itself as a sponsorship recipient, solely in
order to enable the use of https://github.com/sponsors/ipxe as a
central link for sponsorship information.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

.github/FUNDING.yml

@@ -1 +1 @@
-github: [mcb30, NiKiZe]
+github: [ipxe, mcb30, NiKiZe]