patch 9.0.0781: workaround to rename "small" to "smallfont" is clumsy

Problem:    Workaround to rename "small" to "smallfont" is clumsy.
Solution:   Undefine "small" after including windows.h. (Ken Takata)
diff --git a/src/libvterm/include/vterm.h b/src/libvterm/include/vterm.h
index 85fb8e5..e5887c8 100644
--- a/src/libvterm/include/vterm.h
+++ b/src/libvterm/include/vterm.h
@@ -528,7 +528,7 @@
     unsigned int font      : 4; /* 0 to 9 */
     unsigned int dwl       : 1; /* On a DECDWL or DECDHL line */
     unsigned int dhl       : 2; /* On a DECDHL line (1=top 2=bottom) */
-    unsigned int smallfont : 1;
+    unsigned int small     : 1;
     unsigned int baseline  : 2;
 } VTermScreenCellAttrs;