AES-GCM crypto support for SRTP

[ming]

Currently, to support this, you need to do the following:

1. Insert this lines in the "crypto_suites" definition in transport_srtp.c:

       /* cipher AES_GCM */
       {"AEAD_AES_128_GCM", AES_128_GCM, AES_128_GCM_KEYSIZE_WSALT, NULL_AUTH, 0, 16, 16,
          sec_serv_conf},
   

2. Define macro "OPENSSL" in third_party/build/srtp/srtp_config.h, e.g: add this line in the end of the file (before #endif line)

       #define OPENSSL
   

3. Add third_party/srtp/crypto/cipher/aes_gcm_ossl.c file to libsrtp Makefile (third_party/build/srtp/Makefile), e.g:

   ciphers = crypto/cipher/cipher.o crypto/cipher/null_cipher.o      \
             crypto/cipher/aes.o crypto/cipher/aes_icm.o             \
             crypto/cipher/aes_cbc.o crypto/cipher/aes_gcm_ossl.c
   

4. Note that the crypto requires OpenSSL, so make sure OpenSSL is enabled. Also this requires PJSIP 2.5, as this version has newer libsrtp version.