Be explicit with the src prefix in the capnp compile.
Be explicit with the src prefix in the capnp compile.

Looks like in some environments this helps it to be
more robust.

Signed-off-by: Henner Zeller <259ad50fabbfd39355f36ecd6c0afca541ea66ef@acm.org>

chipsalliance/UHDM,chipsalliance/UHDM,chipsalliance/UHDM