mips: add missing declarations

mips: add missing declarations

These are three more functions that are only called from assembler and
only need a declaration to avoid the -Wmissing-prototypes warnings:

arch/mips/kernel/signal.c:904:17: error: no previous prototype for 'do_notify_resume' [-Werror=missing-prototypes]
arch/mips/kernel/traps.c:370:6: error: no previous prototype for 'show_registers' [-Werror=missing-prototypes]
arch/mips/kernel/smp.c:352:17: error: no previous prototype for 'start_secondary' [-Werror=missing-prototypes]

Link: https://lkml.kernel.org/r/20231204115710.2247097-14-arnd@kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Stephen Rothwell <sfr@rothwell.id.au>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>