gnu: xf86-video-openchrome: Patch to correct build failure.

* gnu/packages/xorg.scm (xf86-video-openchrome): Add patch for build failure
    with glibc-2.20. Fixes bug #18740.
* gnu/packages/patches/xf86-video-openchrome-includes.patch: New file.
* gnu-system.am (dist_patch_DATA): Register patch.