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>

tools/perf/builtin-inject.c

@@ -23,6 +23,7 @@
 
 #include <linux/list.h>
 #include <errno.h>
+#include <signal.h>
 
 struct perf_inject {
 	struct perf_tool	tool;