cmake: fix build failure

[razvanazamfirei]

After the addition of libsodium aegis, building fizz fails with the following message:

  Undefined symbols for architecture arm64:
    "_softaes_block_encrypt", referenced from:
        _aegis128l_update in aead_aegis128l_soft.c.o
        _aegis256_encrypt_detached in aead_aegis256_soft.c.o
        _aegis256_decrypt_detached in aead_aegis256_soft.c.o
        _aegis256_update in aead_aegis256_soft.c.o
  ld: symbol(s) not found for architecture arm64
  clang: error: linker command failed with exit code 1 (use -v to see invocation)

It appears to be caused by libsodium-aegis/private/softaes.c not being included in FIZZ_SOURCES. This PR fixes that.

Also see: Homebrew/homebrew-core#133417