perf tools: Introduce filename__read_int helper

Just opens a file and calls atoi() in at most its first 64 bytes.

To read things like /proc/sys/kernel/perf_event_paranoid.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-669q04c5tou5pnt8jtiz6y2r@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/util.h

@@ -305,4 +305,6 @@ struct dso;
 
 char *get_srcline(struct dso *dso, unsigned long addr);
 void free_srcline(char *srcline);
+
+int filename__read_int(const char *filename, int *value);
 #endif /* GIT_COMPAT_UTIL_H */