block/raw-posix: fix compilation warning on OSX

block/raw-posix.c:947:19: warning: unused variable 's' [-Wunused-variable]
    BDRVRawState *s = aiocb->bs->opaque;

This variable is used only when on of the following macros are defined
CONFIG_XFS, CONFIG_FALLOCATE, CONFIG_FALLOCATE_PUNCH_HOLE or
CONFIG_FALLOCATE_ZERO_RANGE. Fortunately, CONFIG_FALLOCATE_PUNCH_HOLE
and CONFIG_FALLOCATE_ZERO_RANGE could be defined only along with
CONFIG_FALLOCATE. Therefore checking for CONFIG_XFS or CONFIG_FALLOCATE
would be enough.

Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Peter Maydell <peter.maydell@linaro.org>
CC: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
