perf tools: Replace %m with %a in sscanf

Not all glibc support %m and it results in a compile error if
%m not supported. Replace it with %a and (float *) casts.

Signed-off-by: Liming Wang <liming.wang@windriver.com>
Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: peterz@infradead.org
Cc: mhiramat@redhat.com
LKML-Reference: <1259743374-9950-1-git-send-email-liming.wang@windriver.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>