libxl: fix Makefile race bug relating to _paths.h

libxl: fix Makefile race bug relating to _paths.h

_paths.h needs to be in AUTOINCS.  That arranges for it to be an
explicit dependency of all object files.  This is necessary so that it
is made before any compilation is attempted.

Making it a dependency of xl.h (as in 25426:e53a1d3c212c) is harmless,
but not sufficient because that only takes effect if there is already
an autogenerated .d file naming xl.h as a dependency of relevant
object files.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>