riscv: traps: add missing prototype

riscv: traps: add missing prototype

Sparse complains:
arch/riscv/kernel/traps.c:213:6: warning: symbol 'shadow_stack' was not declared. Should it be static?

The variable is used in entry.S, so declare shadow_stack there
alongside SHADOW_OVERFLOW_STACK_SIZE.

Fixes: 0ed3552f259f ("riscv: add VMAP_STACK overflow detection")
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20220814141237.493457-5-mail@conchuod.ie
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>