glspectrum: Fixed include bug so that it works correctly on OS X

Signed-off-by: Felix Paul Kühne <fkuehne@videolan.org>

modules/visualization/glspectrum.c

@@ -34,7 +34,11 @@
 #include <vlc_filter.h>
 #include <vlc_rand.h>
 
-#include <GL/gl.h>
+#ifdef __APPLE__
+# include <OpenGL/gl.h>
+#else
+# include <GL/gl.h>
+#endif
 
 #include <math.h>