patch 8.2.3968: build failure

Problem:    Build failure.
Solution:   Add missing changes.
diff --git a/src/strings.c b/src/strings.c
index 18b2cef..6abf89a 100644
--- a/src/strings.c
+++ b/src/strings.c
@@ -916,7 +916,7 @@
 	    {
 		clear_tv(&newtv);
 		clear_tv(&tv);
-		emsg(_(e_stringreq));
+		emsg(_(e_string_required));
 		break;
 	    }
 	    else
@@ -956,7 +956,7 @@
     {
 	if (*p == NUL)
 	{
-	    semsg(_(e_reduceempty), "String");
+	    semsg(_(e_reduce_of_an_empty_str_with_no_initial_value), "String");
 	    return;
 	}
 	if (copy_first_char_to_tv(p, rettv) == FAIL)