perf tools: Add clockid_name function

Add the clockid_name() function to get the clock name based on its
clockid.  It will be used in the following changes.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Geneviève Bastien <gbastien@versatic.net>
Cc: Ian Rogers <irogers@google.com>
Cc: Jeremie Galarneau <jgalar@efficios.com>
Cc: Michael Petlan <mpetlan@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lore.kernel.org/lkml/20200805093444.314999-3-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/clockid.h

@@ -6,4 +6,6 @@
 struct option;
 int parse_clockid(const struct option *opt, const char *str, int unset);
 
+const char *clockid_name(clockid_t clk_id);
+
 #endif