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/option.h b/src/option.h
index aaa9443..254ca6a 100644
--- a/src/option.h
+++ b/src/option.h
@@ -979,7 +979,7 @@
 # define TBIS_GIANT		0x20
 #endif
 EXTERN long	p_ttyscroll;	// 'ttyscroll'
-#if defined(FEAT_MOUSE) && (defined(UNIX) || defined(VMS))
+#if defined(UNIX) || defined(VMS)
 EXTERN char_u	*p_ttym;	// 'ttymouse'
 EXTERN unsigned ttym_flags;
 # define TTYM_XTERM		0x01