Changed `#include` from "xpm.h" to "X11/xpm.h" on `board.c`

This fixes a problem that occurred because "libxpm-dev" was
updated and "xpm.h" is now in "/usr/include/X11/xpm.h".

This commit was originally a patch made by Debian maintainer
Jose Carlos Medeiros <debian@psabs.com.br>, on
"Fri, 28 Apr 2006 19:10:02 -0300".

For further information, check:
http://bugs.debian.org/365165