telemetry: fix build on FreeBSD < 12.2

telemetry: fix build on FreeBSD < 12.2

The function pthread_setname_np() was originally not available on
FreeBSD. It has been added in FreeBSD 12.2:
https://svnweb.freebsd.org/base?view=revision&revision=362264

The EAL implementation of rte_thread_setname() is duplicated
in the telemetry library, which does not depend on EAL,
so the compilation is safe in all systems.

Fixes: 5da7736f8c53 ("telemetry: set socket listener thread name")
Signed-off-by: Thomas Monjalon <thomas@monjalon.net>