Add empty <util.h> to build BSD source.

Hopefully this will let us remove the empty file from bionic. It shouldn't
be there because -- unlike BSD -- we don't actually have a libutil.

Change-Id: I7e3042ca120aafa877709797866b52ac62675870
diff --git a/toolbox/upstream-netbsd/include/util.h b/toolbox/upstream-netbsd/include/util.h
new file mode 100644
index 0000000..e69de29
--- /dev/null
+++ b/toolbox/upstream-netbsd/include/util.h