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

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

Commit b6d40b3c4e4452cee6f7f06d889e4e8d113e9588 removed most uses of the
'fctx' variable from the rctx_dispfail() function: it is now only needed
by the FCTXTRACE3() macro.  However, when --enable-querytrace is not in
effect, that macro evaluates to a list of UNUSED() macros that does not
include "UNUSED(fctx);".  This triggers the following compilation
warning when building without --enable-querytrace:

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

Fix by adding "UNUSED(fctx);" lines to all FCTXTRACE*() macros.  This is
safe to do because all of those macros use the 'fctx' local variable, so
there is no danger of introducing new errors caused by use of undeclared
identifiers.