win32/Makefile.sub: fix typo

Other places are using RT_VER. So I guess this is correct.
Currently AppVeyor CI is failing and I want to fix that.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@62063 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

win32/Makefile.sub

@@ -736,7 +736,7 @@ $(CONFIG_H): $(MKFILES) $(srcdir)/win32/Makefile.sub $(win_srcdir)/Makefile.sub
 #define HAVE_FLOCK 1
 #define HAVE_ISNAN 1
 #define HAVE_FINITE 1
-!if $(RT_VAR) >= 120
+!if $(RT_VER) >= 120
 #define HAVE_NAN 1
 !endif
 #define HAVE_HYPOT 1