ensure gnutls is initialized before use

Section 7.1.2 of the gnutls info manual states that it must be
initialized with a call to gnutls_global_init before use. This
fixes the syslog warning I was seeing:

  lxc-create: Libgcrypt warning: missing initialization -
              please fix the application

Signed-off-by: Dwight Engen <dwight.engen@oracle.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>