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

When building with W=1:

    arch/m68k/kernel/vectors.c:52:13: warning: no previous prototype for ‘base_trap_init’ [-Wmissing-prototypes]
       52 | void __init base_trap_init(void)
	  |             ^~~~~~~~~~~~~~

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

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/bd0a2f65bc1681dc45e2b24951bd89f9ddbe2eef.1694613528.git.geert@linux-m68k.org

arch/m68k/kernel/vectors.c

@@ -28,6 +28,8 @@
 #include <asm/fpu.h>
 #include <asm/traps.h>
 
+#include "vectors.h"
+
 /* assembler routines */
 asmlinkage void system_call(void);
 asmlinkage void buserr(void);

arch/m68k/kernel/vectors.h

@@ -0,0 +1,3 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+
+void base_trap_init(void);