config: build: dont allow STRIP_KERNEL_EXPORTS on kernel 6.6
STRIP_KERNEL_EXPORTS is currently not working on kernel 6.6 as there
have been major changes in the upstream kernel.
I have looked at it, and I dont think we can adapt the current patch to
work so until this is fixed lets prevent STRIP_KERNEL_EXPORTS from
being selected on 6.6.
Link: https://github.com/openwrt/openwrt/pull/15498
Signed-off-by: Robert Marko <[email protected]>