patch 8.2.3546: build failure without the +eval feature

Problem:    Build failure without the +eval feature.
Solution:   Add #ifdef. (closes #9025)
diff --git a/src/errors.h b/src/errors.h
index 96a9b2f..7778b39 100644
--- a/src/errors.h
+++ b/src/errors.h
@@ -160,10 +160,12 @@
 	INIT(= N_("E711: List value does not have enough items"));
 EXTERN char e_cannot_slice_dictionary[]
 	INIT(= N_("E719: Cannot slice a Dictionary"));
+#endif
 EXTERN char e_conflicts_with_value_of_listchars[]
 	INIT(= N_("E834: Conflicts with value of 'listchars'"));
 EXTERN char e_conflicts_with_value_of_fillchars[]
 	INIT(= N_("E835: Conflicts with value of 'fillchars'"));
+#ifdef FEAT_EVAL
 EXTERN char e_assert_fails_second_arg[]
 	INIT(= N_("E856: \"assert_fails()\" second argument must be a string or a list with one or two strings"));
 EXTERN char e_using_invalid_value_as_string_str[]