crypto: fix function signatures for nettle 2.7 vs 3

Nettle version 2.7.x used 'unsigned int' instead of 'size_t' for length
parameters in functions. Use a local typedef so that we can build with
the correct signature depending on nettle version, as we already do in
the cipher code.

Reported-by: Amol Surati <suratiamol@gmail.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>