Add declaration of SSL_get_selected_srtp_profile() for OpenSSL.

Because there was an extra declaration in the vendor version, we locally
removed the second one in r238405 with 1.0.1c. Later, upstream fixed it in
1.0.2d but they removed the first one. Therefore, both were removed in our
version unfortunately. Now we revert to the vendor one to re-add it.

MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D10525