kernel: Add necessary CONFIG_MTD_SPLIT config option

Buils break without this new config, mea culpa.

Link: https://github.com/openwrt/openwrt/pull/21855
Signed-off-by: Linus Walleij <[email protected]>

target/linux/generic/config-6.12

@@ -3922,6 +3922,7 @@ CONFIG_MTD_SPLIT_FIRMWARE_NAME="firmware"
 # CONFIG_MTD_SPLIT_LZMA_FW is not set
 # CONFIG_MTD_SPLIT_MINOR_FW is not set
 # CONFIG_MTD_SPLIT_MSTC_BOOT is not set
+# CONFIG_MTD_SPLIT_OPENWRT_PROLOG is not set
 # CONFIG_MTD_SPLIT_SEAMA_FW is not set
 # CONFIG_MTD_SPLIT_SEIL_FW is not set
 CONFIG_MTD_SPLIT_SQUASHFS_ROOT=y