Revert "Cleanup: Remove unused variables in winstuff.c"

This reverts commit 32a96b80a3.

this needed an ifdef, not a removal, will land a proper fix later

source/blender/blenlib/intern/winstuff.c

@@ -79,6 +79,10 @@ bool BLI_windows_register_blend_extension(const bool background)
   char buffer[256];
 
   char BlPath[MAX_PATH];
+  char InstallDir[FILE_MAXDIR];
+  char SysDir[FILE_MAXDIR];
+  const char *ThumbHandlerDLL;
+  char RegCmd[MAX_PATH * 2];
   char MBox[256];
   char *blender_app;
 #  ifndef _WIN64