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>

dirmngr/t-http.c

@@ -118,7 +118,7 @@ my_gnutls_log (int level, const char *text)
 }
 #endif
 
-
+#if HTTP_USE_NTBTLS
 static gpg_error_t
 my_http_tls_verify_cb (void *opaque,
                        http_t http,
@@ -165,6 +165,7 @@ my_http_tls_verify_cb (void *opaque,
   log_info ("my_http_tls_verify_cb returns: %s\n", gpg_strerror (err));
   return err;
 }
+#endif /*HTTP_USE_NTBTLS*/