riscv: add a forward declaration for cpuinfo_op

Add a forward declaration for cpuinfo_op to resolve a sparse warning.

Link: https://lore.kernel.org/r/b831f349-5d0c-f7ac-8362-acb20bc6221a@kernel.org
Signed-off-by: Paul Walmsley <pjw@kernel.org>

Author: Paul Walmsley

arch/riscv/include/asm/cpufeature.h

@@ -31,6 +31,8 @@ struct riscv_isainfo {
 
 DECLARE_PER_CPU(struct riscv_cpuinfo, riscv_cpuinfo);
 
+extern const struct seq_operations cpuinfo_op;
+
 /* Per-cpu ISA extensions. */
 extern struct riscv_isainfo hart_isa[NR_CPUS];