riscv: kvm: define vcpu_sbi_ext_pmu in header

Sparse is giving a warning about vcpu_sbi_ext_pmu not being
defined, so add a definition to the relevant header to fix
the following:

arch/riscv/kvm/vcpu_sbi_pmu.c:81:37: warning: symbol 'vcpu_sbi_ext_pmu' was not declared. Should it be static?

Fixes: cbddc4c4 ("RISC-V: KVM: Add SBI PMU extension support")
Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Reviewed-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Anup Patel <anup@brainfault.org>