Fixed|Wad Map Converter: Export DP_Initialize as extern "C"

Otherwise, the plugin loader does not recognize it as a valid
Doomsday plugin.

doomsday/plugins/wadmapconverter/src/wadmapconverter.cpp

@@ -292,7 +292,7 @@ int ConvertMapHook(int hookType, int parm, void* context)
  * This function is called automatically when the plugin is loaded.
  * We let the engine know what we'd like to do.
  */
-void DP_Initialize(void)
+extern "C" void DP_Initialize(void)
 {
     Plug_AddHook(HOOK_MAP_CONVERT, ConvertMapHook);
 }