[gtksourceview] build: invert disable assert check

[Christian Hergert <chergert src gnome org>]

commit b77d51b6533e11bad7257da8dc51944e3e37a942
Author: Christian Hergert <chergert redhat com>
Date:   Wed Jan 29 09:25:53 2020 -0800

    build: invert disable assert check

 gtksourceview/gtksourcelanguage-parser-2.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/gtksourceview/gtksourcelanguage-parser-2.c b/gtksourceview/gtksourcelanguage-parser-2.c
index b43262f6..1a0f3939 100644
--- a/gtksourceview/gtksourcelanguage-parser-2.c
+++ b/gtksourceview/gtksourcelanguage-parser-2.c
@@ -1474,7 +1474,7 @@ handle_keyword_char_class_element (ParserState *parser_state)
                return;
 
        do {
-#ifndef G_DISABLE_ASSERT
+#ifdef G_DISABLE_ASSERT
                G_GNUC_UNUSED
 #endif
                int ret;