Trying to compile MD_C without any of the hash modules would result in a
bunch of unused parameter warning (hence errors in -Werror builds).
We could silence those warnings by casting the parameters to void, but
still, compiling the module in such a configuration would mean all of
its functions are useless (always returning an error).
Seems better to just document the dependency.
Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
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