KVM: selftests: Avoid using forced target for generating arm64 headers

The 'prepare' target that generates the arm64 sysreg headers had no
prerequisites, so it wound up forcing a rebuild of all KVM selftests
each invocation. Add a rule for the generated headers and just have
dependents use that for a prerequisite.

Reported-by: Nina Schoetterl-Glausch <nsg@linux.ibm.com>
Fixes: 9697d84c

 ("KVM: selftests: Generate sysreg-defs.h and add to include path")
Tested-by: Nina Schoetterl-Glausch <nsg@linux.ibm.com>
Link: https://lore.kernel.org/r/20231027005439.3142015-3-oliver.upton@linux.dev
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>