engines/http.c: add fallthrough annotation to _curl_trace

engines/http.c: add fallthrough annotation to _curl_trace

To avoid the warning from clang "warning: unannotated fall-through
between switch labels [-Wimplicit-fallthrough]" swap the "fall through"
comment with the "fallthrough;" annotation from compiler.h.

Since the second "fall through" comment isn't really a new fall-through,
remove it.

Signed-off-by: Rebecca Cran <rebecca@bsdio.com>
Link: https://lore.kernel.org/r/20211016061738.76654-1-rebecca@bsdio.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>