RISC-V: Fix build warning for smpboot.c

The following warnings are reported by kbuild with W=1.

>> arch/riscv/kernel/smpboot.c:109:5: warning: no previous prototype for
'start_secondary_cpu' [-Wmissing-prototypes]
     109 | int start_secondary_cpu(int cpu, struct task_struct *tidle)
         |     ^~~~~~~~~~~~~~~~~~~
   arch/riscv/kernel/smpboot.c:146:34: warning: no previous prototype for
'smp_callin' [-Wmissing-prototypes]
     146 | asmlinkage __visible void __init smp_callin(void)
         |                                  ^~~~~~~~~~

Fix the warnings by marking the local functions static and adding the prototype
for the global function.

Signed-off-by: Atish Patra <atish.patra@wdc.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>