patch 8.1.0138: negative value of 'softtabstop' not used correctly

Problem:    Negative value of 'softtabstop' not used correctly.
Solution:   Use get_sts_value(). (Tom Ryder)
diff --git a/src/Makefile b/src/Makefile
index ad90451..86dae88 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -2288,6 +2288,7 @@
 	test_syn_attr \
 	test_syntax \
 	test_system \
+	test_tab \
 	test_tabline \
 	test_tabpage \
 	test_tagcase \