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

When building with W=1:

arch/m68k/kernel/signal.c:756:18: warning: no previous prototype for ‘do_sigreturn’ [-Wmissing-prototypes]
756 | asmlinkage void *do_sigreturn(struct pt_regs *regs, struct switch_stack *sw)
| ^~~~~~~~~~~~
arch/m68k/kernel/signal.c:783:18: warning: no previous prototype for ‘do_rt_sigreturn’ [-Wmissing-prototypes]
783 | asmlinkage void *do_rt_sigreturn(struct pt_regs *regs, struct switch_stack *sw)
| ^~~~~~~~~~~~~~~
arch/m68k/kernel/signal.c:1112:17: warning: no previous prototype for ‘do_notify_resume’ [-Wmissing-prototypes]
1112 | asmlinkage void do_notify_resume(struct pt_regs *regs)
| ^~~~~~~~~~~~~~~~

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

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/25cecda80698829cec18721a9d0f058cc69df0cc.1694613528.git.geert@linux-m68k.org