fix GtkSourceSearchFlags values

changes values from:

	0, 1, 2

to:

	1, 2, 4

before, GTK_SOURCE_SEARCH_VISIBLE_ONLY was 0,
and as the the enum values are used as bit-field,
checking for this bit would always return in _not set_:

    visible_only = (flags & GTK_SOURCE_SEARCH_VISIBLE_ONLY) != 0;

src/gtksourceiter.h

@@ -28,9 +28,9 @@ G_BEGIN_DECLS
 
 typedef enum
 {
-	GTK_SOURCE_SEARCH_VISIBLE_ONLY,
-	GTK_SOURCE_SEARCH_TEXT_ONLY,
-	GTK_SOURCE_SEARCH_CASE_INSENSITIVE
+	GTK_SOURCE_SEARCH_VISIBLE_ONLY = 1,
+	GTK_SOURCE_SEARCH_TEXT_ONLY = 2,
+	GTK_SOURCE_SEARCH_CASE_INSENSITIVE = 4
 	/* Possible future plans: SEARCH_REGEXP */
 } GtkSourceSearchFlags;