kernel: add missing symbol NET_DSA_TAG_RTL4_A

Fix build breakage caused by the missing kernel config symbol.

Fixes: 8735997686a3 ("kernel: backport RTL8366RB patches")
Signed-off-by: Hannu Nyman <[email protected]>

target/linux/generic/config-5.4

@@ -3442,6 +3442,7 @@ CONFIG_NET_CORE=y
 # CONFIG_NET_DSA_TAG_LAN9303 is not set
 # CONFIG_NET_DSA_TAG_MTK is not set
 # CONFIG_NET_DSA_TAG_QCA is not set
+# CONFIG_NET_DSA_TAG_RTL4_A is not set
 # CONFIG_NET_DSA_TAG_SJA1105 is not set
 # CONFIG_NET_DSA_TAG_TRAILER is not set
 # CONFIG_NET_DSA_VITESSE_VSC73XX is not set