BUILD: xprt-quic: replace ERR_func_error_string() with ERR_peek_error_func()

OpenSSL 3.0 warns that ERR_func_error_string() is deprecated. Using
ERR_peek_error_func() solves it instead, and this function was added to
the compat layer by commit 1effd9aa0 ("MINOR: ssl: Remove call to
ERR_func_error_string with OpenSSLv3").
diff --git a/src/xprt_quic.c b/src/xprt_quic.c
index 63cfc7f..d675d16 100644
--- a/src/xprt_quic.c
+++ b/src/xprt_quic.c
@@ -2004,14 +2004,16 @@
 {
 	if (unlikely(global.mode & MODE_DEBUG)) {
 		while (1) {
+			const char *func = NULL;
 			unsigned long ret;
 
+			ERR_peek_error_func(&func);
 			ret = ERR_get_error();
 			if (!ret)
 				return;
 
 			fprintf(stderr, "conn. @%p OpenSSL error[0x%lx] %s: %s\n", conn, ret,
-			        ERR_func_error_string(ret), ERR_reason_error_string(ret));
+			        func, ERR_reason_error_string(ret));
 		}
 	}
 }