ixp4xx: add 6.18 testing kernel

Allow for selecting v6.18 it as a testing kernel. One single
Kconfig option needed adding.

Link: openwrt/openwrt#22896
Signed-off-by: Linus Walleij <linusw@kernel.org>

Author: linusw

target/linux/ixp4xx/Makefile

@@ -12,6 +12,7 @@ CPU_TYPE:=xscale
 SUBTARGETS:=generic
 
 KERNEL_PATCHVER:=6.12
+KERNEL_TESTING_PATCHVER:=6.18
 
 define Target/Description
 	Build firmware images for the IXP4xx XScale CPU