m68k: arch/m68k/kernel/sun3-head.S needs <linux/init.h>

Fix this:

  arch/m68k/kernel/sun3-head.S: Assembler messages:
  arch/m68k/kernel/sun3-head.S:32: Error: Unknown operator -- statement `__head' ignored

Introduced by commit 6f335cab

 ("m68k:
convert to use __HEAD and HEAD_TEXT macros."), which started using
__HEAD without adding the appropriate include.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>