Fix a MSVC compiler error.

The problem was reported and the patch tested by YogiHH. I didn't
check whether the scoped_recource really needs a complete type, just
changed it for MSVC.

src/floating_textbox.cpp

@@ -19,7 +19,6 @@
 #include "game_display.hpp"
 #include "game_preferences.hpp"
 #include "log.hpp"
-#include "widgets/textbox.hpp"
 
 #include <ctime>
 

src/floating_textbox.hpp

@@ -18,6 +18,8 @@
 
 #include "global.hpp"
 
+// Scoped_resource can't use a pointer to an incomplete pointer with MSVC.
+#include "widgets/textbox.hpp"
 #include "scoped_resource.hpp"
 
 #include <string>
@@ -29,7 +31,6 @@ class unit_map;
 
 namespace gui{
 
-	class textbox;
 	class button;
 
 	enum TEXTBOX_MODE { TEXTBOX_NONE, TEXTBOX_SEARCH, TEXTBOX_MESSAGE,