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

When building with W=1:

arch/m68k/kernel/traps.c:754:17: warning: no previous prototype for ‘buserr_c’ [-Wmissing-prototypes]
754 | asmlinkage void buserr_c(struct frame *fp)
| ^~~~~~~~
arch/m68k/kernel/traps.c:1140:17: warning: no previous prototype for ‘set_esp0’ [-Wmissing-prototypes]
1140 | asmlinkage void set_esp0(unsigned long ssp)
| ^~~~~~~~
arch/m68k/kernel/traps.c:1155:17: warning: no previous prototype for ‘fpemu_signal’ [-Wmissing-prototypes]
1155 | asmlinkage void fpemu_signal(int signal, int code, void *addr)
| ^~~~~~~~~~~~
arch/m68k/kernel/traps.c:1149:17: warning: no previous prototype for ‘fpsp040_die’ [-Wmissing-prototypes]
1149 | asmlinkage void fpsp040_die(void)
| ^~~~~~~~~~~

Fix this by introducing a new header file "traps.h" for holding the
prototypes of functions implemented in 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/652cbbb1c9e339980a86068ebdd0a69362324af8.1694613528.git.geert@linux-m68k.org