dirmngr: Avoid warnings during non-ntbtls build.

* dirmngr/t-http.c (my_http_tls_verify_cb): Avoid warnings when not
using ntbtls.

Without this patch, when building without ntbtls, we see the following
warnings during "make check":

t-http.c: In function ‘my_http_tls_verify_cb’:
t-http.c:141:16: warning: implicit declaration of function

‘ntbtls_x509_get_peer_cert’ [-Wimplicit-function-declaration]
  (cert = ntbtls_x509_get_peer_cert (tls_context, idx)); idx++)
          ^~~~~~~~~~~~~~~~~~~~~~~~~

t-http.c:141:14: warning: assignment makes pointer from integer

without a cast  -Wint-conversion]
 (cert = ntbtls_x509_get_peer_cert (tls_context, idx)); idx++)
       ^

At top level:
t-http.c:123:1: warning: ‘my_http_tls_verify_cb’ defined but not

used [-Wunused-function]

my_http_tls_verify_cb (void *opaque,
^~~~~~~~~~~~~~~~~~~~~

• Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>