stun: Rename a symbol to avoid a naming clash with OpenSSL

stun: Rename a symbol to avoid a naming clash with OpenSSL

The symbol is not exported, but nevertheless leaks in the static version
of libnice, due to limitations on symbol visibilities with static
libraries. OpenSSL has a symbol named RAND_bytes(), which ours clashes
with if the two are linked together statically.

Avoid this by prefixing ours with ‘nice_’.