gnu: libxi: Fix CVE-2016-{7945,7946}.

* gnu/packages/patches/libxi-CVE-2016-7945-CVE-2016-7946.patch: New file.
* gnu/local.mk (dist_patch_DATA): Add it.
* gnu/packages/xorg.scm (libxi)[replacement]: New field.
(libxi/fixed): New variable.