m68k: kernel: Include <linux/cpu.h> for trap_init()

When building with W=1:

arch/m68k/kernel/vectors.c:74:13: warning: no previous prototype for ‘trap_init’ [-Wmissing-prototypes]
74 | void __init trap_init (void)
| ^~~~~~~~~

Fix this by including <linux/cpu.h>.

Signed-off-by: Geert Uytterhoeven <[e-mail]>
Acked-by: Arnd Bergmann <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

arch/m68k/kernel/vectors.c

···
 /*
  * Sets up all exception vectors
  */
+#include <linux/cpu.h>
 #include <linux/sched.h>
 #include <linux/kernel.h>
 #include <linux/linkage.h>