Use vector for varobj_list_children interface.

	Use vector for varobj_list_children interface.
        * gdb/varobj.c (varobj_list_children): Return vector
        of varobjs.
        * gdb/varobj.h (varobj_list_children): Adjust
        prototype.
        (varobj_p): Declare.  Declare vector thereof.
        * mi/mi-cmd-var.c (mi_cmd_var_list_children): Adjust
        for varobj_list_children change.
	* Makefile.in (varobj_h): Update dependencies.