Fix compilation after removal of TRUE macro

This macro was removed by qtbase/158f39ec7870c54ff3a15f72fe151062d6daa172
but it seems like it was misused anyways. This function should return
a mng_bool (MNG_TRUE) instead.

Change-Id: I6f390e32faec18c52e5d7f659015cea8be4d27a5
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>

src/plugins/imageformats/mng/qmnghandler.cpp

@@ -98,7 +98,7 @@ static mng_bool myerror(mng_handle /*hMNG*/,
         (iChunkname>>8)&0xff,
         (iChunkname>>0)&0xff,
         iExtra1,iExtra2);
-    return TRUE;
+    return MNG_TRUE;
 }
 
 static mng_ptr myalloc(mng_size_t iSize)