patch 8.2.3251: listing builtin_gui as an available terminal is confusing

Problem:    Listing builtin_gui as an available terminal is confusing.
Solution:   Do not list builtin_gui. (Christian Brabandt, closes #8669,
            closes #8661)
diff --git a/src/term.c b/src/term.c
index cec16b6..a13959b 100644
--- a/src/term.c
+++ b/src/term.c
@@ -1786,7 +1786,8 @@
     mch_errmsg("\r\n");
     for (termp = &(builtin_termcaps[0]); termp->bt_string != NULL; ++termp)
     {
-	if (termp->bt_entry == (int)KS_NAME)
+	if (termp->bt_entry == (int)KS_NAME
+		&& STRCMP(termp->bt_string, "gui") != 0)
 	{
 #ifdef HAVE_TGETENT
 	    mch_errmsg("    builtin_");