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>