RISC-V: cpu_ops_spinwait.c should include head.h

RISC-V: cpu_ops_spinwait.c should include head.h

Running sparse shows cpu_ops_spinwait.c is missing two definitions
found in head.h, so include it to stop the following warnings:

arch/riscv/kernel/cpu_ops_spinwait.c:15:6: warning: symbol '__cpu_spinwait_stack_pointer' was not declared. Should it be static?
arch/riscv/kernel/cpu_ops_spinwait.c:16:6: warning: symbol '__cpu_spinwait_task_pointer' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben.dooks@sifive.com>
Link: https://lore.kernel.org/r/20220713215306.94675-1-ben.dooks@sifive.com
Fixes: c78f94f35cf6 ("RISC-V: Use __cpu_up_stack/task_pointer only for spinwait method")
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>