To enable linking of a current libXrender libNX_X11 needs that symbol.

This is a (manual) backport of the following upstream libX11 commit
available at
https://cgit.freedesktop.org/xorg/lib/libX11/commit/src/XlibInt.c?id=4a060f993bf676cf21ad9784e010f54134da7b40:

     Commit: 4a060f993bf676cf21ad9784e010f54134da7b40
     Author: Peter Hutterer <peter.hutterer@who-t.net>
     Date: Mon, 17 Oct 2011 09:45:15 +1000
     Subject: Add _XGetRequest as substitute for GetReq/GetReqExtra