MIPS: dec: Declare which_prom() as static

MIPS: dec: Declare which_prom() as static

Declare which_prom() as static to suppress gcc compiler warning that
'missing-prototypes'. This function is not intended to be called
from other parts.

Fix follow error with gcc-14 when -Werror:

arch/mips/dec/prom/init.c:45:13: error: no previous prototype for ‘which_prom’ [-Werror=missing-prototypes]
   45 | void __init which_prom(s32 magic, s32 *prom_vec)
      |             ^~~~~~~~~~
cc1: all warnings being treated as errors
make[6]: *** [scripts/Makefile.build:207: arch/mips/dec/prom/init.o] Error 1
make[5]: *** [scripts/Makefile.build:465: arch/mips/dec/prom] Error 2
make[5]: *** Waiting for unfinished jobs....

Signed-off-by: WangYuli <wangyuli@uniontech.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>