crypto: Allow building with GnuTLS but without Libtasn1

crypto: Allow building with GnuTLS but without Libtasn1

We only use Libtasn1 in unit tests. As noted in commit d47b83b118
("tests: add migration tests of TLS with x509 credentials"), having
GnuTLS without Libtasn1 is a valid configuration, so do not require
Libtasn1, to avoid:

  Dependency gnutls found: YES 3.7.1 (cached)
  Run-time dependency libtasn1 found: NO (tried pkgconfig)

  ../meson.build:1914:10: ERROR: Dependency "libtasn1" not found, tried pkgconfig

Fixes: ba7ed407e6 ("configure, meson: convert libtasn1 detection to meson")
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>