ARM: KVM: define KVM_ARM_MAX_VCPUS unconditionally

The CONFIG_KVM_ARM_MAX_VCPUS symbol is needed in order to build the
kernel/context_tracking.c code, which includes the vgic data structures
implictly through the kvm headers. Definining the symbol to zero
on builds without KVM resolves this build error:

In file included from include/linux/kvm_host.h:33:0,
                 from kernel/context_tracking.c:18:
arch/arm/include/asm/kvm_host.h:28:23: warning: "CONFIG_KVM_ARM_MAX_VCPUS" is not defined [-Wundef]
 #define KVM_MAX_VCPUS CONFIG_KVM_ARM_MAX_VCPUS
                       ^
arch/arm/include/asm/kvm_vgic.h:34:24: note: in expansion of macro 'KVM_MAX_VCPUS'
 #define VGIC_MAX_CPUS  KVM_MAX_VCPUS
                        ^
arch/arm/include/asm/kvm_vgic.h:38:6: note: in expansion of macro 'VGIC_MAX_CPUS'
 #if (VGIC_MAX_CPUS > 8)
      ^
In file included from arch/arm/include/asm/kvm_host.h:41:0,
                 from include/linux/kvm_host.h:33,
                 from kernel/context_tracking.c:18:
arch/arm/include/asm/kvm_vgic.h:59:11: error: 'CONFIG_KVM_ARM_MAX_VCPUS' undeclared here (not in a function)
  } percpu[VGIC_MAX_CPUS];
           ^

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Christoffer Dall <cdall@cs.columbia.edu>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Christoffer Dall <cdall@cs.columbia.edu>