build: avoid gcrypt deprecation warnings

build: avoid gcrypt deprecation warnings

When combining old gcc (4.2.1) and new gcrypt (1.5.2), such as
when using the Ports repository on FreeBSD, the build fails with:

  CC       libvirt_driver_la-libvirt.lo
cc1: warnings being treated as errors
In file included from libvirt.c:58:
/usr/local/include/gcrypt.h:1336: warning: 'gcry_ac_io_mode_t' is deprecated [-Wdeprecated-declarations]

Relevant part of gcrypt.h:
1333 typedef struct gcry_ac_io
1334 {
1335   /* This is an INTERNAL structure, do NOT use manually.  */
1336   gcry_ac_io_mode_t mode _GCRY_ATTR_INTERNAL;
1337   gcry_ac_io_type_t type _GCRY_ATTR_INTERNAL;
1338   union

The sad part is that we aren't even using the deprecated symbols - their
mere inclusion in the installed header is provoking the problems.  It
looks like newer gcc is a bit more tolerant (that is, this is a
shortcoming of FreeBSD's use of an older compiler).

Signed-off-by: Eric Blake <eblake@redhat.com>