libmd: add dependency workaround for r366344

r366344 fixed and reenabled the assembly optimized skein implementation,
but skein_block objects were not being rebuilt in no-clean builds. This
resulted in failing no-clean builds. SKEIN_USE_ASM controls which
routines come from C vs assembly, and with no explicit dependency
r366344's change to SKEIN_USE_ASM did not cause skein_block.{o,pico}
to be rebuilt.

Add a dependency on this Makefile for the skein_block objects. This
dependency is broader in scope than absolutely required (that is, the
skein_block objects will now be rebuilt on any change to this Makefile).
There are ways this could be addressed, but it is probably not worth the
additional effort or testing time to pursue them.

PR: 248221
Reported by: kevans, Jeremy Faulkner
Discussed with: kevans
Sponsored by: The FreeBSD Foundation