perf trace: Fix build when DWARF unwind isn't available

perf trace: Fix build when DWARF unwind isn't available

The variable is initialized and then conditionally set to a different
value, but not used when DWARF unwinding is not available, bummer, write
1000 times: "Run make -C tools/perf build-test"...

  builtin-trace.c: In function ‘cmd_trace’:
  builtin-trace.c:3112:6: error: variable ‘max_stack_user_set’ set but not
  used [-Werror=unused-but-set-variable]
    bool max_stack_user_set = true;
        ^
  cc1: all warnings being treated as err

Fix it by marking it as __maybe_unused.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Milian Wolff <milian.wolff@kdab.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Fixes: 056149932602 ("perf trace: Make --(min,max}-stack imply "--call-graph dwarf"")
Link: http://lkml.kernel.org/n/tip-85r40c5hhv6jnmph77l1hgsr@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>