m68k: sun3x: Make sun3x_halt() static

When building with W=1:

    arch/m68k/sun3x/prom.c:33:6: warning: no previous prototype for ‘sun3x_halt’ [-Wmissing-prototypes]
       33 | void sun3x_halt(void)
	  |      ^~~~~~~~~~

Fix this by making sun3x_halt() static.
The function body was moved to arch/m68k/sun3x/prom.c in v2.4.5.2.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/0ba2883aaff2e4fc5e570bfee87c58e483668b26.1694613528.git.geert@linux-m68k.org