Always enable wxMBConv::IsUTF8()

These where previously guarded by wxUSE_UNICODE_UTF8 but
may be useful in other configurations too.

interface/wx/strconv.h

@@ -68,6 +68,13 @@ public:
     */
     static size_t GetMaxMBNulLen();
 
+    /**
+        return true if the converter's charset is UTF-8, i.e. char* strings
+        decoded using this object can be directly copied to wxString's internal
+        storage without converting to WC and than back to UTF-8 MB string
+    */
+    virtual bool IsUTF8() const;
+
     /**
         Convert multibyte string to a wide character one.