perf bench futex: Use __maybe_unused

perf bench futex: Use __maybe_unused

Instead of attributing a variable to itself to silence the compiler, use
the attribute designed for that, avoiding this:

In file included from bench/futex-hash.c:24:
bench/futex.h:95:7: error: explicitly assigning value of variable of type 'pthread_attr_t *' to itself [-Werror,-Wself-assign]
        attr = attr;
        ~~~~ ^ ~~~~
bench/futex.h:96:13: error: explicitly assigning value of variable of type 'size_t' (aka 'unsigned long') to itself [-Werror,-Wself-assign]
        cpusetsize = cpusetsize;
        ~~~~~~~~~~ ^ ~~~~~~~~~~
bench/futex.h:97:9: error: explicitly assigning value of variable of type 'cpu_set_t *' (aka 'struct cpu_set_t *') to itself [-Werror,-Wself-assign]
        cpuset = cpuset;
        ~~~~~~ ^ ~~~~~~

That is only triggered when HAVE_PTHREAD_ATTR_SETAFFINITY_NP isn't set.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-14ws1d1elj2d5ej8g7cwdqau@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>