riscv: kvm: move extern sbi_ext declarations to a header

Sparse complains about missing statics in the declarations of several
variables:
arch/riscv/kvm/vcpu_sbi_replace.c:38:37: warning: symbol 'vcpu_sbi_ext_time' was not declared. Should it be static?
arch/riscv/kvm/vcpu_sbi_replace.c:73:37: warning: symbol 'vcpu_sbi_ext_ipi' was not declared. Should it be static?
arch/riscv/kvm/vcpu_sbi_replace.c:126:37: warning: symbol 'vcpu_sbi_ext_rfence' was not declared. Should it be static?
arch/riscv/kvm/vcpu_sbi_replace.c:170:37: warning: symbol 'vcpu_sbi_ext_srst' was not declared. Should it be static?
arch/riscv/kvm/vcpu_sbi_base.c:69:37: warning: symbol 'vcpu_sbi_ext_base' was not declared. Should it be static?
arch/riscv/kvm/vcpu_sbi_base.c:90:37: warning: symbol 'vcpu_sbi_ext_experimental' was not declared. Should it be static?
arch/riscv/kvm/vcpu_sbi_base.c:96:37: warning: symbol 'vcpu_sbi_ext_vendor' was not declared. Should it be static?
arch/riscv/kvm/vcpu_sbi_hsm.c:115:37: warning: symbol 'vcpu_sbi_ext_hsm' was not declared. Should it be static?

These variables are however used in vcpu_sbi.c where they are declared
as extern. Move them to kvm_vcpu_sbi.h which is handily already
included by the three other files.

Fixes: a046c2d8 ("RISC-V: KVM: Reorganize SBI code by moving SBI v0.1 to its own file")
Fixes: 5f862df5 ("RISC-V: KVM: Add v0.1 replacement SBI extensions defined in v0.2")
Fixes: 3e1d8656

 ("RISC-V: KVM: Add SBI HSM extension in KVM")
Reviewed-by: Palmer Dabbelt <palmer@rivosinc.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Anup Patel <anup@brainfault.org>