log: add missing experimental tag

log: add missing experimental tag

When rte_log_register_type_and_pick_level() has been introduced, it has
been correctly added to the EXPERIMENTAL section of the eal map and the
symbol itself has been marked at its definition.

However, the declaration of this symbol in rte_log.h is missing the
__rte_experimental tag.
Because of this, a user can try to call this symbol without being aware
this is an experimental api (neither compilation nor link warning).

Fixes: b22e77c02620 ("eal: register log type and pick level from args")
Cc: stable@dpdk.org
Signed-off-by: David Marchand <david.marchand@redhat.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>