Currently the file list generated by Doxygen has src/ at the
beginning of each path.  Paths like common/sysdefs.h and
liblzma/api/lzma.h are easier to read without such a prefix.

Builds from a separate build directory with

	mkdir build
	cd build
	../configure
	doxygen Doxyfile

include an even longer prefix /home/someone/src/xz/src; this
patch has the nice side-effect of eliminating that prefix, too.

Fixes: http://bugs.debian.org/572273