Functions from ed25519_core are needed for GNUnet to build.
Include them in the minimal build of libsodium so we don't need to
switch to the full build just for that.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>

scalarmult_ed25519_ref10 was excluded from 'minimal' variant (which is
what we are building) by upstream commit f5076db5f8ef27.
Revert that to allow downstream projects to make use of it (eg. gnunet)

Signed-off-by: Daniel Golle <daniel@makrotopia.org>