OCL: fixed warning

see http://build.opencv.org/builders/macos_x64%5Bdebug%5D--%5Bsse_%3D_ON%5D--%5Btbb_%3D_OFF%5D--%5Bgpu_%3D_OFF%5D--%5Bcompiler_%3D_gcc%5D--%5Btests_%3D_ON%5D-/builds/1111/steps/compile/logs/warnings%20%281%29