KVM: arm64: Add missing #include of <linux/bitmap.h> to kvm_host.h

kvm_host.h uses DECLARE_BITMAP() to declare the features member of
struct vcpu_arch, but the corresponding #include for this is
missing.

This patch adds a suitable #include for <linux/bitmap.h>.  Although
the header builds without it today, this should help to avoid
future surprises.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>