lavf: Add functions for SRTP decryption/encryption

This supports the AES_CM_128_HMAC_SHA1_80 and
AES_CM_128_HMAC_SHA1_32 cipher suites (from RFC 4568) at the
moment. The main missing features are replay protection (which can be
added later without changing the internal API), and the F8 and null
ciphers.

Signed-off-by: Martin Storsjö <martin@martin.st>