realtek: add v6.18 as testing kernel

Add kernel v6.18 as testing kernel for realtek target.

Signed-off-by: Jonas Jelonek <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/21181
Signed-off-by: Robert Marko <[email protected]>

target/linux/realtek/Makefile

@@ -10,6 +10,7 @@ FEATURES:=ramdisk squashfs
 SUBTARGETS:=rtl838x rtl839x rtl930x rtl930x_nand rtl931x rtl931x_nand
 
 KERNEL_PATCHVER:=6.12
+KERNEL_TESTING_PATCHVER:=6.18
 
 define Target/Description
 	Build firmware images for Realtek RTL83xx/RTL93xx based boards.