Add "UNUSED(fctx);" to FCTXTRACE*() macro stubs

This fixes a build issue introduced by #3133 (closed)/!5810 (merged)/!5857 (merged) that was caught overnight by pairwise testing:

• https://gitlab.isc.org/isc-projects/bind9/-/jobs/2306000
• https://gitlab.isc.org/isc-projects/bind9/-/jobs/2306293

To reproduce, just run:

autoreconf -fi && ./configure --enable-warn-error && make

(We run almost all builds in GitLab CI with --enable-developer, which turns on --enable-querytrace and that was masking the problem.)

resolver.c: In function 'rctx_dispfail':
resolver.c:7888:21: warning: unused variable 'fctx' [-Wunused-variable]
 7888 |         fetchctx_t *fctx = rctx->fctx;
      |                     ^~~~

See #3133 (closed)