perf tools: Add signal.h to places using its definitions

And remove it from util.h, disentangling it a bit more.

Link: http://lkml.kernel.org/n/tip-2zg9s5nx90yde64j3g4z2uhk@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
diff --git a/tools/perf/builtin-stat.c b/tools/perf/builtin-stat.c
index be2cd53..e3837fe 100644
--- a/tools/perf/builtin-stat.c
+++ b/tools/perf/builtin-stat.c
@@ -70,6 +70,7 @@
 #include <linux/time64.h>
 #include <api/fs/fs.h>
 #include <errno.h>
+#include <signal.h>
 #include <stdlib.h>
 #include <sys/prctl.h>
 #include <inttypes.h>