perf stat: Do not print ratio when task-clock event is not counted

The ratio between the number of events and the time elapsed makes
sense only if task-clock event is counted. Otherwise it will be
simply a (confusing)

	#      0.000 M/sec

This patch outputs the ratio only if task-clock event is counted.
Some test examples of before and after:

Before:

 [lucas@skywalker linux.trees.git]$ sudo perf stat -e branch-misses -a -- sleep 1

	 Performance counter stats for 'sleep 1':

		1367818  branch-misses            #      0.000 M/sec

	    1.001494325  seconds time elapsed

After (without task-clock):

 [lucas@skywalker perf]$ sudo ./perf stat -e branch-misses -a -- sleep 1

	 Performance counter stats for 'sleep 1':

		1135044  branch-misses

	    1.001370775  seconds time elapsed

After (with task-clock):

 [lucas@skywalker perf]$ sudo ./perf stat -e branch-misses -e task-clock -a -- sleep 1

	 Performance counter stats for 'sleep 1':

		1070111  branch-misses            #      0.534 M/sec
	    2002.730893  task-clock-msecs         #      1.999 CPUs

	    1.001640292  seconds time elapsed

Signed-off-by: Lucas De Marchi <lucas.de.marchi@gmail.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
LKML-Reference: <20091115140507.GB21561@skywalker.lan>
Signed-off-by: Ingo Molnar <mingo@elte.hu>