KVM: arm64: avoid unused-variable warning

KVM: arm64: avoid unused-variable warning

The 'cpu' variable is only used inside of an #ifdef block and causes
a warning if there is no user:

arch/arm64/kvm/hyp_trace.c: In function 'kvm_hyp_trace_init':
arch/arm64/kvm/hyp_trace.c:422:13: error: unused variable 'cpu' [-Werror=unused-variable]
  422 |         int cpu;
      |             ^~~

Change the #ifdef to an equivalent IS_ENABLED() check to avoid
the warning.

Fixes: b22888917fa4 ("KVM: arm64: Sync boot clock with the nVHE/pKVM hyp")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Vincent Donnefort <vdonnefort@google.com>
Link: https://patch.msgid.link/20260313094925.3749287-1-arnd@kernel.org
Signed-off-by: Marc Zyngier <maz@kernel.org>