meson: Propagate gnutls dependency to migration

meson: Propagate gnutls dependency to migration

Commit 3eacf70bb5a83e4775ad8003cbca63a40f70c8c2 neglected to fix this
for softmmu configs, which pull in migration's use of gnutls.

This fixes the following compilation failure on Arm-based Macs:

  In file included from migration/multifd.c:23:
  In file included from migration/tls.h:25:
  In file included from include/io/channel-tls.h:26:
  In file included from include/crypto/tlssession.h:24:
  include/crypto/tlscreds.h:28:10: fatal error: 'gnutls/gnutls.h' file not found
  #include <gnutls/gnutls.h>
           ^~~~~~~~~~~~~~~~~
  1 error generated.

(as well as for channel.c and tls.c)

Signed-off-by: Jessica Clarke <jrtc27@jrtc27.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Message-Id: <20210320171221.37437-1-jrtc27@jrtc27.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>