perf timechart: Use NSEC_PER_U?SEC

perf timechart: Use NSEC_PER_U?SEC

Following kernel practices, using linux/time64.h

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Stanislav Fomichev <stfomichev@yandex-team.ru>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-5l1md8lsdhfnrlsqyejzo9w2@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>