OCppunitTest_sc_opencl_test: only depend on scopencl if ENABLE_OPENCL.

scopencl is only built when ENABLE_OPENCL is true, but the unit test
should still run successfully using the fallback mechanism without
scopencl (i.e. when --disable-opencl is used).

Change-Id: I4b44148f1f59ad8b3d9c78c2fd0e1cbe2030db37