MAINTAINERS: add selftests to network drivers

tools/testing/selftests/drivers/net/ is not listed under
networking entries. Add it to NETWORKING DRIVERS.

Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20240814142832.3473685-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: kuba-moo

MAINTAINERS

@@ -15886,6 +15886,7 @@ F:	include/linux/netdevice.h
 F:	include/uapi/linux/cn_proc.h
 F:	include/uapi/linux/if_*
 F:	include/uapi/linux/netdevice.h
+F:	tools/testing/selftests/drivers/net/
 X:	drivers/net/wireless/
 
 NETWORKING DRIVERS (WIRELESS)