win_defs.h: redefine RGB macro after undefining

Before this change, if os_defs.h was included after macros.h then
win_defs.h would undefine our own RGB macro.

vim-patch:8.0.0146

src/nvim/version.c

@@ -806,7 +806,7 @@ static const int included_patches[] = {
   // 149,
   // 148,
   // 147,
-  // 146,
+  146,
   // 145 NA
   // 144 NA
   // 143,