build: fix ngtcp2 crypto library detection

[jay]

• Change library link check for ngtcp2_crypto_{gnutls,openssl} to
  to use function ngtcp2_crypto_recv_client_initial_cb instead of
  ngtcp2_crypto_ctx_initial.

The latter function is no longer external since two days ago in
ngtcp2/ngtcp2@533451f. curl HTTP/3 CI builds have been failing since
then because they would not link to the ngtcp2 crypto library.

Ref: ngtcp2/ngtcp2#356

Closes #xxxx

---

See this zuul build of curl-novalgrind-ngtcp2-gnutls for example:

  CCLD     curl
../lib/.libs/libcurl.so: undefined reference to `ngtcp2_crypto_recv_retry_cb'
../lib/.libs/libcurl.so: undefined reference to `ngtcp2_crypto_recv_crypto_data_cb'
../lib/.libs/libcurl.so: undefined reference to `ngtcp2_crypto_decrypt_cb'
...and so on

/cc @tatsuhiro-t

[bagder]

#7522 is truly annoying...