patch 8.2.0816: terminal test fails when compiled with Athena

Problem:    Terminal test fails when compiled with Athena.
Solution:   Do give an error when the GUI is not running. (hint by Dominique
            Pelle, closes #5928, closes #6132)
diff --git a/src/globals.h b/src/globals.h
index 60c3c66..648da7d 100644
--- a/src/globals.h
+++ b/src/globals.h
@@ -1767,6 +1767,9 @@
 
 EXTERN char e_lock_unlock[]	INIT(= N_("E940: Cannot lock or unlock variable %s"));
 #endif
+#if defined(FEAT_GUI) || defined(FEAT_TERMGUICOLORS)
+EXTERN char e_alloc_color[]	INIT(= N_("E254: Cannot allocate color %s"));
+#endif
 
 #ifdef FEAT_GUI_MAC
 EXTERN short disallow_gui	INIT(= FALSE);