[libkwineffects] Try fixing compile error with clang

The FreeBSD build is currently failing with:

kwingltexture_p.h:83:23: error: no function named 'cleanupGL' with type
'void ()' was found in the specified scope
friend void KWin::cleanupGL();

This change hopefully addresses the problem.

libkwineffects/kwingltexture_p.h

@@ -24,6 +24,7 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 #define KWIN_GLTEXTURE_P_H
 
 #include "kwinconfig.h" // KWIN_HAVE_OPENGL
+#include "kwinglutils.h"
 #include <kwinglutils_export.h>
 
 #include <QSize>