erlang-p1-tls: indicate OpenSSL exception in license info

The license of erlang-p1-tls is indeed GPLv2+ as indicated in
ERLANG_P1_TLS_LICENSE, but this GPLv2+ carries an OpenSSL exception,
which allows it to be linked against the OpenSSL library. Therefore,
we update the license information accordingly.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>