Platform: fix warning about undeclared function.

src/Platform/NaClApplication.h

@@ -475,6 +475,7 @@ When no other application header is included this macro is also aliased to
 /* look at that insane placement of __attribute__. WTF. */
 #define MAGNUM_NACLAPPLICATION_MAIN(application)                            \
     namespace pp {                                                          \
+        Module __attribute__ ((visibility ("default"))) * CreateModule();   \
         Module __attribute__ ((visibility ("default"))) * CreateModule() {  \
             return new Magnum::Platform::Implementation::NaClModule<application>(); \
         }                                                                   \