m68k: kernel: Add and use "ints.h"

When building with W=1:

arch/m68k/kernel/ints.c:165:17: warning: no previous prototype for ‘handle_badint’ [-Wmissing-prototypes]
165 | asmlinkage void handle_badint(struct pt_regs *regs)
| ^~~~~~~~~~~~~

Fix this by introducing a new header file "ints.h" for holding the
prototypes of functions implemented in arch/m68k/kernel/ints.c.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/dc65d01ca4c7de94ce814e5b5e1f726fff97566b.1694613528.git.geert@linux-m68k.org