selftests: hsr: add missing config for CONFIG_BRIDGE

hsr_redbox.sh test need to create bridge for testing. Add the missing
config CONFIG_BRIDGE in config file.

Fixes: eafbf057 ("test: hsr: Extend the hsr_redbox.sh to have more SAN devices connected")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Tested-by: Simon Horman <horms@kernel.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>