m68k: coldfire: add and use "vectors.h"

When building with W=1:

    arch/m68k/coldfire/vectors.c:43:13: warning: no previous prototype for ‘trap_init’ [-Wmissing-prototypes]
     void __init trap_init(void)
                 ^~~~~~~~~

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

Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>

arch/m68k/coldfire/vectors.c

@@ -18,6 +18,8 @@
 #include <asm/mcfsim.h>
 #include <asm/mcfwdebug.h>
 
+#include "vectors.h"
+
 /***************************************************************************/
 
 #ifdef TRAP_DBG_INTERRUPT

arch/m68k/coldfire/vectors.h

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