* src/Input/input.hxx

MSVC fix from Bernie Bright:

You can keep the enum private if you add the following declarations
immediately afterwards:

  struct mouse;
  friend struct mouse;

It seems that MSVC doesn't grant the nested mouse decl. any special
access privileges to its surrounding class.

src/Input/input.hxx

@@ -248,6 +248,8 @@ private:
     MAX_MICE = 1,
     MAX_MOUSE_BUTTONS = 8
   };
+  struct mouse;
+  friend struct mouse;
 
   typedef vector<FGBinding *> binding_list_t;