perf tools: Fix memory leak in read_ftrace_printk()

get_tracing_file() should be paired with put_tracing_file().

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
LKML-Reference: <4AB1F48F.4070807@cn.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>