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>