m68k: 68000: fix warnings in 68000 interrupt handling

When building with W=1:

      CC      arch/m68k/68000/ints.o
    arch/m68k/68000/ints.c:77:6: warning: no previous prototype for ‘process_int’ [-Wmissing-prototypes]
     void process_int(int vec, struct pt_regs *fp)
          ^~~~~~~~~~~
    arch/m68k/68000/ints.c:153:13: warning: no previous prototype for ‘trap_init’ [-Wmissing-prototypes]
     void __init trap_init(void)
                 ^~~~~~~~~

Include linux/cpu.h to get the prototype for taps_init().
Create a local ints.h for prototype of process_int(). Also mark
process_int() as asmlinkage, since it is called from the first level
interrupt assembly handler.

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