winsys/sw/xlib: fix compile error in xlib_sw_winsys.c.

winsys/sw/xlib: fix compile error in xlib_sw_winsys.c.

xlib_sw_winsys.h:5:22: fatal error: X11/Xlib.h: No such file or directory

The compiler cannot find the Xlib.h in the installed system headers.
All supplied include directives point to inside the mesa module.
The X11_CFLAGS variable is undefined (not defined in config.status).

It appears the intent was to use X11_INCLUDES defined in configure.ac.

The Xlib.h file is not installed on my workstation. It is supplied in
the libx11-dev package. This allows an X developer control over which
version of this file is used for X development.

Signed-off-by: Gaetan Nadon <memsize@videotron.ca>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>