tools lib api fs: Adopt filename__read_str from perf

We already moved similar functions in here, also it'll be useful for
sysfs__read_str addition in following patch.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1455465826-8426-3-git-send-email-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/lib/api/fs/fs.h

@@ -2,6 +2,7 @@
 #define __API_FS__
 
 #include <stdbool.h>
+#include <unistd.h>
 
 /*
  * On most systems <limits.h> would have given us this, but  not on some systems
@@ -26,6 +27,7 @@ FS(tracefs)
 
 int filename__read_int(const char *filename, int *value);
 int filename__read_ull(const char *filename, unsigned long long *value);
+int filename__read_str(const char *filename, char **buf, size_t *sizep);
 
 int sysctl__read_int(const char *sysctl, int *value);
 int sysfs__read_int(const char *entry, int *value);