Update min required crypto library versions

The min required versions for crypto libraries are now

 - gnutls >= 3.1.18
 - nettle >= 2.7.1
 - gcrypt >= 1.5.0

# gpg: Signature made Fri 19 Oct 2018 14:42:35 BST
# gpg:                using RSA key BE86EBB415104FDF
# gpg: Good signature from "Daniel P. Berrange <dan@berrange.com>"
# gpg:                 aka "Daniel P. Berrange <berrange@redhat.com>"
# Primary key fingerprint: DAF3 A6FD B26B 6291 2D0E  8E3F BE86 EBB4 1510 4FDF

* remotes/berrange/tags/qcrypto-next-pull-request:
  crypto: require nettle >= 2.7.1 for building QEMU
  crypto: require libgcrypt >= 1.5.0 for building QEMU
  crypto: require gnutls >= 3.1.18 for building QEMU
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>