m68k: sun3: Make sun3_platform_init() static

When building with W=1:

    arch/m68k/sun3/config.c:201:12: warning: no previous prototype for ‘sun3_platform_init’ [-Wmissing-prototypes]
      202 | int __init sun3_platform_init(void)
	  |            ^~~~~~~~~~~~~~~~~~

Fix this by making sun3_platform_init() static.
There was never a user outside arch/m68k/sun3/config.c.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/f35e6376d01b11f21f677cb980093e8410d2c33b.1694613528.git.geert@linux-m68k.org