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/proto/term.pro b/src/proto/term.pro
index 2091cbf..4ac2858 100644
--- a/src/proto/term.pro
+++ b/src/proto/term.pro
@@ -1,6 +1,7 @@
 /* term.c */
 guicolor_T termgui_get_color(char_u *name);
 guicolor_T termgui_mch_get_rgb(guicolor_T color);
+void set_color_count(int nr);
 int set_termname(char_u *term);
 void getlinecol(long *cp, long *rp);
 int add_termcap_entry(char_u *name, int force);