Use _WIN32 instead of WIN32 to detect Windows

`WIN32` is not defined when building a 64-bit executable on Windows with Clang. `_WIN32` is the more appropriate option here.
http://web.archive.org/web/20191012035921/http://nadeausoftware.com/articles/2012/01/c_c_tip_how_use_compiler_predefined_macros_detect_operating_system contains a handy table/matrix of the options and why this is best.

rtgui/dirbrowser.h

@@ -72,7 +72,7 @@ private:
 
     bool expandSuccess;
 
-#ifdef WIN32
+#ifdef _WIN32
     unsigned int volumes;
 public:
     void updateVolumes ();