build: Fix make distcheck

I forgot to delete the underscore in object_locking_SOURCES when
changing the name in one of previous cleanups.
Signed-off-by: NMartin Kletzander <mkletzan@redhat.com>

tests/Makefile.am

@@ -1016,7 +1016,7 @@ CILOPTINCS =
 CILOPTPACKAGES = -package unix,str,cil
 CILOPTLIBS = -linkpkg
 
-object_locking_SOURCES = objectlocking.ml
+objectlocking_SOURCES = objectlocking.ml
 
 %.cmx: %.ml
 	ocamlfind ocamlopt $(CILOPTFLAGS) $(CILOPTINCS) $(CILOPTPACKAGES) -c $<