cmake: Fix dependency issue in x86 build scripts

Incremental builds have been broken in x86 due to a misconfigured
dependency. mmu_tables.bin is always generated, even for "nothing to
do" builds.

We fix this by removing the stray dependency on user_mmu_tables.bin
when not CONFIG_X86_KPTI.

Steps to reproduce:

Build any sample twice with qemu_x86 and observe that the second build
regenerates mmu_tables.bin.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>