Fix macOS 10.13 build error with HAVE_UNISTD_H having no value.

In file included from tests/util/piglit-util-cl.c:26:
In file included from tests/util/piglit-util-cl.h:29:
In file included from tests/util/piglit-util-cl-enum.h:34:
In file included from /System/Library/Frameworks/OpenCL.framework/Headers/opencl.h:16:
In file included from /System/Library/Frameworks/OpenCL.framework/Headers/gcl.h:23:
/usr/include/dispatch/dispatch.h:38:45: error: expected value in expression
#if !defined(HAVE_UNISTD_H) || HAVE_UNISTD_H
                                            ^

Signed-off-by: Vinson Lee <vlee@freedesktop.org>