m68k: kernel: Make bad_super_trap() static

When building with W=1:

    arch/m68k/kernel/traps.c:968:6: warning: no previous prototype for ‘bad_super_trap’ [-Wmissing-prototypes]
      968 | void bad_super_trap (struct frame *fp)
	  |      ^~~~~~~~~~~~~~

Fix this by making bad_super_trap() static.
There was never a user outside arch/m68k/kernel/traps.c.
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/1e0b67a355d7e1bcb40811eced41d3080e8f4d20.1694613528.git.geert@linux-m68k.org

arch/m68k/kernel/traps.c

@@ -965,7 +965,7 @@ void show_stack(struct task_struct *task, unsigned long *stack,
  * real 68k parts, but it won't hurt either.
  */
 
-void bad_super_trap (struct frame *fp)
+static void bad_super_trap(struct frame *fp)
 {
 	int vector = (fp->ptregs.vector >> 2) & 0xff;