Document wxDynamicLibrary::GetDllExt() method

Make this method part of the public API.

Closes https://github.com/wxWidgets/wxWidgets/pull/273

interface/wx/dynlib.h

@@ -117,6 +117,16 @@ public:
     */
     wxDynamicLibrary(const wxString& name, int flags = wxDL_DEFAULT);
 
+    /**
+        Returns the platform-specific dynamic library file extension, or
+        depending on @a flags, the plugin file extension. The leading dot
+        is included.
+
+        For example, on Windows @c ".dll" is returned, and either @c ".dylib"
+        or @c ".bundle" on OS X.
+    */
+    static wxString GetDllExt(wxDynamicLibraryCategory cat = wxDL_LIBRARY);
+
     /**
         Returns the platform-specific full name for the library called @a name.
         E.g. it adds a @c ".dll" extension under Windows and @c "lib" prefix