oprofile: fix oprofile regression: select RING_BUFFER_ALLOW_SWAP

commit 85bac32c
    ring-buffer: only enable ring_buffer_swap_cpu when needed
broke oprofile (at least on s390, but likely on all platforms).

this patch lets oprofile select RING_BUFER_ALLOW_SWAP to make
ring_buffer_swap_cpu usable for oprofile.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
LKML-Reference: <200909162156.49239.borntraeger@de.ibm.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Robert Richter <robert.richter@amd.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>