tools/ocaml: simplify compile/link of test apps

tools/ocaml: simplify compile/link of test apps

xtl doesn't require the full LDLIBS_libxenctrl, just the -L and
xenlight.cmxa, the latter which contains LDLIBS_libxenctrl as needed.
Fixing this avoids the need to be concerned about LDLIBS_libxenctrl
becoming more than one word in the future.

Since the tests are pure ocaml (no C components) CFLAGS and
LIBS_xenlight are not required.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: David Scott <dave@recoil.org>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Cc: David Scott <dave@recoil.org>