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 <geert@linux-m68k.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/42e7055fab50deda1f7cd648982e90b7ab28fdc3.1694613528.git.geert@linux-m68k.org

arch/m68k/kernel/vectors.c

@@ -17,6 +17,7 @@
 /*
  * Sets up all exception vectors
  */
+#include <linux/cpu.h>
 #include <linux/sched.h>
 #include <linux/kernel.h>
 #include <linux/linkage.h>