ocaml-xen-lowlevel-libs: Remove hardcoded dependency on libxl-headers

libxl-headers is a fixup package needed on Ubuntu, where the
xen-devel packages don't include the libxl headers.   It should
not be in the dependency list in the spec file - we should instead
add it when mapping package names in makedeb.

Signed-off-by: Euan Harris <euan.harris@citrix.com>