crypto: Remove use of GCRYPT_VERSION macro.

crypto: Remove use of GCRYPT_VERSION macro.

According to the gcrypt documentation it's intended that
gcry_check_version() is called with the minimum version of gcrypt
needed by the program, not the version from the <gcrypt.h> header file
that happened to be installed when qemu was compiled.  Indeed the
gcrypt.h header says that you shouldn't use the GCRYPT_VERSION macro.

This causes the following failure:

  qemu-img: Unable to initialize gcrypt

if a slightly older version of libgcrypt is installed with a newer
qemu, even though the slightly older version works fine.  This can
happen with RPM packaging which uses symbol versioning to determine
automatically which libgcrypt is required by qemu, which caused the
following bug in RHEL 8:

  https://bugzilla.redhat.com/show_bug.cgi?id=1840485

qemu actually requires libgcrypt >= 1.5.0, so we might put the string
"1.5.0" here.  However since 1.5.0 was released in 2011, it hardly
seems we need to check that.  So I replaced GCRYPT_VERSION with NULL.
Perhaps in future if we move to requiring a newer version of gcrypt we
could put a literal string here.

Signed-off-by: Richard W.M. Jones <rjones@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>