MAINTAINERS: Update ibmveth maintainer

Add Nick Child as the maintainer of the IBM Power Virtual Ethernet
Device Driver, replacing Cristobal Forno.

Signed-off-by: Nick Child <nnac123@linux.ibm.com>
Link: https://lore.kernel.org/r/20220803155246.39582-1-nnac123@linux.ibm.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>