Include wx/string.h from the files that use wxString

Don't rely on this header being implicitly included by wx/object.h even
though it does include it right now.

No real changes.

include/wx/accel.h

@@ -16,6 +16,7 @@
 #if wxUSE_ACCEL
 
 #include "wx/object.h"
+#include "wx/string.h"
 
 class WXDLLIMPEXP_FWD_CORE wxAcceleratorTable;
 class WXDLLIMPEXP_FWD_CORE wxMenuItem;

include/wx/module.h

@@ -12,6 +12,7 @@
 #define _WX_MODULE_H_
 
 #include "wx/object.h"
+#include "wx/string.h"
 #include "wx/vector.h"
 
 class wxModule;

src/common/encconv.cpp

@@ -12,6 +12,7 @@
 
 
 #include "wx/encconv.h"
+#include "wx/string.h"
 
 #include <stdlib.h>