examples/performance-thread: fix build with clang 12.0.1

examples/performance-thread: fix build with clang 12.0.1

In clang 12.0.1 version, the use of pthread_yield() is deprecated,
use sched_yield() instead.

log:
    examples/performance-thread/pthread_shim/main.c:75:9: warning:
    'pthread_yield' is deprecated: pthread_yield is deprecated,
    use sched_yield instead [-Wdeprecated-declarations]

Bugzilla ID: 745
Fixes: 433ba6228f9a ("examples/performance-thread: add pthread_shim app")
Cc: stable@dpdk.org
Signed-off-by: Jerin Jacob <jerinj@marvell.com>
Tested-by: Ali Alnubani <alialnu@nvidia.com>