generic: 5.15: add missing symbol

Found during apm821xx refresh.

Signed-off-by: Christian Lamparter <[email protected]>

target/linux/generic/config-5.15

@@ -4117,6 +4117,7 @@ CONFIG_NET_SCH_FQ_CODEL=y
 # CONFIG_NET_SCH_TBF is not set
 # CONFIG_NET_SCH_TEQL is not set
 # CONFIG_NET_SCTPPROBE is not set
+# CONFIG_NET_SELFTESTS is not set
 CONFIG_NET_SOCK_MSG=y
 # CONFIG_NET_SWITCHDEV is not set
 # CONFIG_NET_TCPPROBE is not set