nbd/server: Mark variable unused in nbd_negotiate_meta_queries

From clang-13:
nbd/server.c:976:22: error: variable 'bitmaps' set but not used \
    [-Werror,-Wunused-but-set-variable]

which is incorrect; see //bugs.llvm.org/show_bug.cgi?id=3888.

Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>