mips: move build_tlb_refill_handler() prototype

Instead of having a declaration for each caller, have one that is shared
with the function definition, which avoids a warning:

arch/mips/mm/tlbex.c:2547:6: error: no previous prototype for 'build_tlb_refill_handler' [-Werror=missing-prototypes]

Link: https://lkml.kernel.org/r/20231204115710.2247097-8-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>