src/Makefile: Fix parallel build after xen_xl_disk parser introduction

src/Makefile: Fix parallel build after xen_xl_disk parser introduction

Well, the parallel build doesn't work as there are not dependencies
set correctly. When running 'make -j' I see this error:

make[2]: Entering directory '/home/zippy/work/libvirt/libvirt.git/src'
  GEN      util/virkeymaps.h
  GEN      locking/lock_protocol.h
make[2]: *** No rule to make target 'xenconfig/xen_xl_disk.h', needed by 'all'.  Stop.
make[2]: *** Waiting for unfinished jobs....
  GEN      lxc/lxc_controller_dispatch.h

The fix is to correctly set dependencies by letting make know that .c
and .h are to be generated from .l. Moreover, the section is moved
closer to the other section which uses it.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Pavel Hrdina <phrdina@redhat.com>