m68k: mac: Make mac_platform_init() static

When building with W=1:

    arch/m68k/mac/config.c:961:12: warning: no previous prototype for ‘mac_platform_init’ [-Wmissing-prototypes]
      961 | int __init mac_platform_init(void)
	  |            ^~~~~~~~~~~~~~~~~

Fix this by making mac_platform_init() static.
There was never a user outside arch/m68k/mac/config.c.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/f5f069f94c369b7de223bb5e1a21837807fde982.1694613528.git.geert@linux-m68k.org