BUILD: quic: Implicit conversion between SSL related enums.

BUILD: quic: Implicit conversion between SSL related enums.

Fix such compilation issues:

include/haproxy/quic_tls.h:157:10: error: implicit conversion from
'enum ssl_encryption_level_t' to 'enum quic_tls_enc_level'
[-Werror=enum-conversion]
  157 |   return ssl_encryption_application;
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~
src/xprt_quic.c: In function 'quic_conn_enc_level_init':
src/xprt_quic.c:2358:13: error: implicit conversion from
'enum quic_tls_enc_level' to 'enum ssl_encryption_level_t'
[-Werror=enum-conversion]
 2358 |  qel->level = quic_to_ssl_enc_level(level);
      |             ^

Not detected by all the compilators.