selftests: tc-testing: enable CONFIG_NET_SCH_RED as a module

a proper kernel configuration for running kselftest can be obtained with:

 $ yes | make kselftest-merge

enable compile support for the 'red' qdisc: otherwise, tdc kselftest fail
when trying to run tdc test items contained in red.json.

Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Link: https://lore.kernel.org/r/cfa23f2d4f672401e6cebca3a321dd1901a9ff07.1606416464.git.dcaratti@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/tc-testing/config

@@ -59,6 +59,7 @@ CONFIG_NET_IFE_SKBPRIO=m
 CONFIG_NET_IFE_SKBTCINDEX=m
 CONFIG_NET_SCH_FIFO=y
 CONFIG_NET_SCH_ETS=m
+CONFIG_NET_SCH_RED=m
 
 #
 ## Network testing