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.
diff --git a/include/haproxy/quic_tls.h b/include/haproxy/quic_tls.h
index 2db41dd..01821a7 100644
--- a/include/haproxy/quic_tls.h
+++ b/include/haproxy/quic_tls.h
@@ -144,7 +144,7 @@
 }
 
 /* These two following functions map our encryption level to the TLS implementation ones. */
-static inline enum quic_tls_enc_level quic_to_ssl_enc_level(enum quic_tls_enc_level level)
+static inline enum ssl_encryption_level_t quic_to_ssl_enc_level(enum quic_tls_enc_level level)
 {
 	switch (level) {
 	case QUIC_TLS_ENC_LEVEL_INITIAL: