Fix assimp_qt_viewer build failed on OSX

tools/assimp_qt_viewer/glview.cpp

@@ -6,7 +6,11 @@
 #include "glview.hpp"
 
 // Header files, OpenGL.
-#include <GL/glu.h>
+#if defined(__APPLE__)
+# include <OpenGL/glu.h>
+#else
+# include <GL/glu.h>
+#endif
 
 // Header files, DevIL.
 #include <il.h>