selftests: net: add missing required classifier

the udpgro_fraglist self-test uses the BPF classifiers, but the
current net self-test configuration does not include it, causing
CI failures:

 # selftests: net: udpgro_frglist.sh
 # ipv6
 # tcp - over veth touching data
 # -l 4 -6 -D 2001:db8::1 -t rx -4 -t
 # Error: TC classifier not found.
 # We have an error talking to the kernel
 # Error: TC classifier not found.
 # We have an error talking to the kernel

Add the missing knob.

Fixes: edae34a3 ("selftests net: add UDP GRO fraglist + bpf self-tests")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Maciej Żenczykowski <maze@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/7c3643763b331e9a400e1874fe089193c99a1c3f.1706170897.git.pabeni@redhat.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/config

@@ -51,6 +51,7 @@ CONFIG_NETFILTER_XT_MATCH_LENGTH=m
 CONFIG_NET_ACT_CT=m
 CONFIG_NET_ACT_GACT=m
 CONFIG_NET_CLS_BASIC=m
+CONFIG_NET_CLS_BPF=m
 CONFIG_NET_CLS_U32=m
 CONFIG_NET_IPGRE_DEMUX=m
 CONFIG_NET_IPGRE=m