perf tools: Add missing headers, mostly stdlib.h

Part of the erosion of util/util.h, that will lose its include stdlib.h,
we need to add it to places where it is needed but was getting it
indirectly.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-1imnqezw99ahc07fjeb51qby@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-help.c

@@ -16,6 +16,7 @@
 #include <linux/kernel.h>
 #include <errno.h>
 #include <stdio.h>
+#include <stdlib.h>
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <unistd.h>