patch 8.2.0554: the GUI doesn't set t_Co

Problem:    The GUI doesn't set t_Co.
Solution:   In the GUI set t_Co to 256 * 256 * 256. (closes #5903)
diff --git a/src/term.c b/src/term.c
index 6afecba..ce935dc 100644
--- a/src/term.c
+++ b/src/term.c
@@ -1552,7 +1552,7 @@
  * Store it as a number in t_colors.
  * Store it as a string in T_CCO (using nr_colors[]).
  */
-    static void
+    void
 set_color_count(int nr)
 {
     char_u	nr_colors[20];		// string for number of colors