MIPS: idle: Add prototype for function check_wait

This commit adds a prototype to fix warning at W=1:

  arch/mips/kernel/idle.c:126:13: error: no previous prototype for 'check_wait' [-Werror=missing-prototypes]

Signed-off-by: Pujin Shi <shipujin.t@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>