build: fix xenconfig VPATH builds

Ever since commit 2c78051a 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: NEric Blake <eblake@redhat.com>

src/Makefile.am

 ## Process this file with automake to produce Makefile.in
 
-## Copyright (C) 2005-2014 Red Hat, Inc.
+## Copyright (C) 2005-2015 Red Hat, Inc.
 ##
 ## This library is free software; you can redistribute it and/or
 ## modify it under the terms of the GNU Lesser General Public
@@ -1074,6 +1074,7 @@ if WITH_XENCONFIG
 # Add the generated object to its own library to control CFLAGS
 noinst_LTLIBRARIES += libvirt_xenxldiskparser.la
 libvirt_xenxldiskparser_la_CFLAGS = \
+		-I$(srcdir)/xenconfig \
 		-I$(top_srcdir)/src/conf $(AM_CFLAGS) -Wno-unused-parameter
 libvirt_xenxldiskparser_la_SOURCES = \
 	$(XENXLDISKPARSER_SOURCES)