test/app_overhead: fix aio make issue

AIO not working because the check for $OS==Linux
is before the $OS is set. Therefore, the binary
build does not include libaio

Change-Id: Iab862d0113d637134a5275d46252c60fe27dde5e
Signed-off-by: John Kariuki <John.K.Kariuki@intel.com>
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/4516


Reviewed-by: Mellanox Build Bot
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>
Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Community-CI: Broadcom CI
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>