perf daemon: Use sig_atomic_t to avoid UB

Use sig_atomic_t for a variable written to in a signal handler and
read elsewhere. This is undefined behavior as per:

https://wiki.sei.cmu.edu/confluence/display/c/SIG31-C.+Do+not+access+shared+objects+in+signal+handlers

Signed-off-by: Ian Rogers <[e-mail]>
Cc: Adrian Hunter <[e-mail]>
Cc: Alexander Shishkin <[e-mail]>
Cc: Alexey Bayduraev <[e-mail]>
Cc: German Gomez <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Leo Yan <[e-mail]>
Cc: Mark Rutland <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Stephane Eranian <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/builtin-daemon.c

···
 	NULL
 };
 
-static bool done;
+static volatile sig_atomic_t done;
 
 static void sig_handler(int sig __maybe_unused)
 {