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

arch/m68k/kernel/signal.c

@@ -51,6 +51,8 @@
 #include <asm/ucontext.h>
 #include <asm/cacheflush.h>
 
+#include "signal.h"
+
 #ifdef CONFIG_MMU
 
 /*

arch/m68k/kernel/signal.h

+/* SPDX-License-Identifier: GPL-2.0-only */
+
+#include <linux/linkage.h>
+
+asmlinkage void do_notify_resume(struct pt_regs *regs);
+asmlinkage void *do_sigreturn(struct pt_regs *regs, struct switch_stack *sw);
+asmlinkage void *do_rt_sigreturn(struct pt_regs *regs, struct switch_stack *sw);