[make] Fix CSDP build in split num configuration.

It seems that recent fixes using ocamlbuild to locate num missed to
switch the csdpcert binary. This is needed to solve problems like the
one in this issue: ocaml/opam-repository#11316

Makefile.build

@@ -530,7 +530,7 @@ CSDPCERTCMO:=lib/clib.cma $(addprefix plugins/micromega/, \
 
 $(CSDPCERT): $(call bestobj, $(CSDPCERTCMO))
 	$(SHOW)'OCAMLBEST -o $@'
-	$(HIDE)$(call bestocaml,,nums unix)
+	$(HIDE)$(call bestocaml, -package num,unix)
 
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 # tests