m68k: include asm/cmpxchg.h in our m68k atomic.h

After commit 9ffc93f203c18a70623f21950f1dd473c9ec48cd ("Remove all

  CC      init/main.o
In file included from include/linux/mm.h:15:0,
                 from include/linux/ring_buffer.h:5,
                 from include/linux/ftrace_event.h:4,
                 from include/trace/syscall.h:6,
                 from include/linux/syscalls.h:78,
                 from init/main.c:16:
include/linux/debug_locks.h: In function ‘__debug_locks_off’:
include/linux/debug_locks.h:16:2: error: implicit declaration of function ‘xchg’

There is no indirect inclusions of the new asm/cmpxchg.h for m68k here.
Looking at most other architectures they include asm/cmpxchg.h in their
asm/atomic.h. M68k currently does not do this. Including this in atomic.h
fixes all m68k build problems.

Signed-off-by: Greg Ungerer <gerg@uclinux.org>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>