mips: signal: move sigcontext declarations to header

Function declarations should be in a shared header to ensure the
prototypes match the definition:

arch/mips/kernel/signal.c:439:5: error: no previous prototype for 'setup_sigcontext' [-Werror=missing-prototypes]
arch/mips/kernel/signal.c:516:5: error: no previous prototype for 'restore_sigcontext' [-Werror=missing-prototypes]

Link: https://lkml.kernel.org/r/20231204115710.2247097-6-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>