app/testpmd: fix stdout flush after printing stats

app/testpmd: fix stdout flush after printing stats

User can specify stats period(n). The statistics should be available
to user every n second. But the print_stats() function does not
force stdout to be flushed, so for instance, a user reading testpmd's
stdout through pipe will not be able to read it until the stdout
buffer is filled.

Fixes: cfea1f3048d1 ("app/testpmd: print statistics periodically")
Cc: stable@dpdk.org
Signed-off-by: Igor Romanov <igor.romanov@oktetlabs.ru>
Signed-off-by: Andrew Rybchenko <arybchenko@solarflare.com>
Acked-by: Bernard Iremonger <bernard.iremonger@intel.com>