examples/kni: fix build with pkg-config

examples/kni: fix build with pkg-config

rm -f build/kni build/kni-static build/kni-shared
test -d build && rmdir -p build || true
[...]
/usr/bin/ld: /tmp/cc72ssnK.o: undefined reference to symbol
'pthread_join@@GLIBC_2.2.5'

This example explicitly call pthread API and should be linked against
the pthread library.

Fixes: 724beb913b44 ("examples/kni: monitor and update link state continually")
Cc: stable@dpdk.org
Signed-off-by: David Marchand <david.marchand@redhat.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>