avoid redefining xmalloc and xfree.

this file defines them with a parameter, but Xthreads.h (which comes from
xorgproto) only defines them as names, causing conflicting definitions.

fixes build with libX11 1.8.4.

initial import of libX11-1.7.0

initial import of libX11-1.5.99.902

initial import of libX11-1.3.3

initial import of libX11-1.2

initial import of libX11-1.1.4