specl: ensure lyaml is built before running specl.

Parallel make gets carried away without a full and complete
dependency graph, so be sure to specify that specl doesn't work
until lyaml.la is built.
* src/specl.mk (src_specl_DEPS): Add yaml/lyaml.la.
Reported by Reuben Thomas at https://github.com/gvvaughan/specl/issues/1.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>