build: fix xenconfig VPATH builds

build: fix xenconfig VPATH builds

Ever since commit 2c78051 split out a helper library for the sake of
changing CFLAGS, a VPATH build with xenconfig enabled has failed:

  CC       xenconfig/libvirt_xenxldiskparser_la-xen_xl_disk.lo
  ../../src/xenconfig/xen_xl_disk.l:37:21: fatal error: xen_xl.h: No such file or directory
   # include "xen_xl.h"
             ^
  compilation terminated.
  Makefile:9462: recipe for target 'xenconfig/libvirt_xenxldiskparser_la-xen_xl_disk.lo' failed

The solution is to tell the build to look for xen_xl.h relative
to $(srcdir), since we keep that file under version control.

[Not fixed here - the raw use of -Wno-unused-parameter in CFLAGS
is NOT portable; ideally, we should be doing a configure test
and only supplying that argument when we know the compiler supports
-Wunused-parameter; but that's a patch for another day]

[Not fixed here - there are still issues with parallel builds hitting
a race between generating the files and trying to compile/distribute
them]

* src/Makefile.am (libvirt_xenxldiskparser_la_CFLAGS): Add another
include directory.

Signed-off-by: Eric Blake <eblake@redhat.com>