[BLOCK] Fix failing compile with BLK_DEV_IO_TRACE=n

[BLOCK] Fix failing compile with BLK_DEV_IO_TRACE=n

I get a compilation error in sglist-arch branch
with BLK_DEV_IO_TRACE=n:

  CC      block/compat_ioctl.o
/usr0/export/dev/bharrosh/git/pub/linux-2.6-block/block/compat_ioctl.c: In
function ?compat_blk_trace_setup?:
/usr0/export/dev/bharrosh/git/pub/linux-2.6-block/block/compat_ioctl.c:568:
error: expected expression before ?do?
make[2]: *** [block/compat_ioctl.o] Error 1

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>