tools: ccan_depends requires a config.h

Commit a77cc282 causes tools/create-ccan-tree to fail, as it can't build
tools/ccan_depends (via `make tools/ccan_depends`) without a
configuration header present.

This change adds a dependency on config.h, so that the configurator
kicks-in prior to building ccan_depends.o.

Other tools don't need this dependency, as tools/ccan_depends is built
during the Makefile 'include' process.

Signed-off-by: Jeremy Kerr <jeremy.kerr@canonical.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>