Remove dllexport declaration from a nested struct.

Try a blind fix for VC6 linking problems in DLL build related to
wxFileTypeInfo ctor. This is unlikely to fix it but DLL-exporting a nested
struct which, moreover, has only inline methods, seems unnecessary and it just
might help too.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64971 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/mimetype.h

@@ -137,7 +137,7 @@ public:
     // NB: This is a helper to get implicit conversion of variadic ctor's
     //     fixed arguments into something that can be passed to VarArgInit().
     //     Do not use, it's used by the ctor only.
-    struct WXDLLIMPEXP_BASE CtorString
+    struct CtorString
     {
         CtorString(const char *str) : m_str(str) {}
         CtorString(const wchar_t *str) : m_str(str) {}