enable AES-256 with all versions of libSRTP 1.5.x

[traud]

libSRTP API of the cipher type in is not fully symmetric:

• for AES-GCM, you have to specify the bit length as well, like AES_256_GCM
• for AES-ICM, instead of AES_256_ICM, you have to go for AES_ICM

With AES_256_ICM the following call chain
srtp_create(.)
→ srtp_add_stream(.)
→ → srtp_stream_alloc(.)
→ → → → crypto_kernel_alloc_cipher(.) returns err_status_fail = 1
→ → → → → crypto_kernel_get_cipher_type(.) returns NULL = 0
because this cipher type is not registered. AES_ICM as value works, because such a cipher type is registered. That software bug was fixed with cisco/libsrtp@b8cb577. However, that change was not backported to the libSRTP 1.5.x branch, yet. Even then, libSRTP 1.5.0 till 1.5.4 keep that software bug. If you use the symbol AES_ICM for all ICM based crypto suites, AES_128_GCM, and AES_256_GCM, you are compatible with all versions of libSRTP 1.5.x.

Here in this repository, this example is used as a guide – a role model – for libSRTP users. Therefore, please, consider this change for inclusion to avoid non-working downstream projects.