crypto: require nettle >= 2.7.1 for building QEMU

crypto: require nettle >= 2.7.1 for building QEMU

nettle 2.7.1 was released in 2013 and all the distros that are build
target platforms for QEMU [1] include it:

  RHEL-7: 2.7.1
  Debian (Stretch): 3.3
  Debian (Jessie): 2.7.1
  OpenBSD (ports): 3.4
  FreeBSD (ports): 3.4
  OpenSUSE Leap 15: 3.4
  Ubuntu (Xenial): 3.2
  macOS (Homebrew): 3.4

Based on this, it is reasonable to require nettle >= 2.7.1 in QEMU
which allows for some conditional version checks in the code to be
removed.

[1] https://qemu.weilnetz.de/doc/qemu-doc.html#Supported-build-platforms

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>