patch 8.1.2171: mouse support not always available

Problem:    Mouse support not always available.
Solution:   Enable mouse support also in tiny version.  Do not define
            FEAT_MOUSE_XTERM on MS-Windows (didn't really work).
diff --git a/src/optiondefs.h b/src/optiondefs.h
index 47f17ed..b557771 100644
--- a/src/optiondefs.h
+++ b/src/optiondefs.h
@@ -2655,7 +2655,7 @@
 			    (char_u *)&p_tf, PV_NONE,
 			    {(char_u *)FALSE, (char_u *)0L} SCTX_INIT},
     {"ttymouse",    "ttym", P_STRING|P_NODEFAULT|P_NO_MKRC|P_VI_DEF,
-#if defined(FEAT_MOUSE) && (defined(UNIX) || defined(VMS))
+#if defined(UNIX) || defined(VMS)
 			    (char_u *)&p_ttym, PV_NONE,
 #else
 			    (char_u *)NULL, PV_NONE,