tools/ocaml: fix build dependency target

tools/ocaml: fix build dependency target

They are two competiting spelling for the variable holding the path to
"tools/ocaml", $(TOPLEVEL) and $(OCAML_TOPLEVEL). The "Makefile.rules"
which is included in all ocaml Makefiles have one rule which make use
of that variable which is then sometime unset. When building
"ocaml/xenstored", make isn't capable of generating ".ocamldep.make"
because $(TOPLEVEL) isn't defined in this case.

This can fail with an error like this when paths.ml have been
regenerated:
    Error: Files define.cmx and paths.cmx
       make inconsistent assumptions over interface Paths

This patch fix ".ocamldep.make" rule by always spelling the variable
$(OCAML_TOPLEVEL).

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Christian Lindig <christian.lindig@citrix.com>