examples: add flush after stats printing

examples: add flush after stats printing

When printf()'s stdout is line-buffered for terminal, it is fully
buffered for pipes. So, stdout listener can only get the output
when it is flushed (on program termination, when buffer is filled or
manual flush).

stdout buffer might fill slowly since every stats report could be small.

Also when it is fully filled it might contain a part of the last stats
report which makes it very inconvenient for any automation which reads
and parses the output.

Fixes: af75078fece3 ("first public release")
Cc: stable@dpdk.org
Signed-off-by: Georgiy Levashov <georgiy.levashov@oktetlabs.ru>
Signed-off-by: Andrew Rybchenko <arybchenko@solarflare.com>