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: 05614993 ("perf trace: Make --(min,max}-stack imply "--call-graph dwarf"")
Link: http://lkml.kernel.org/n/tip-85r40c5hhv6jnmph77l1hgsr@git.kernel.orgSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-trace.c

@@ -3109,7 +3109,7 @@ int cmd_trace(int argc, const char **argv, const char *prefix __maybe_unused)
 			"per thread proc mmap processing timeout in ms"),
 	OPT_END()
 	};
-	bool max_stack_user_set = true;
+	bool __maybe_unused max_stack_user_set = true;
 	bool mmap_pages_user_set = true;
 	const char * const trace_subcommands[] = { "record", NULL };
 	int err;