perf annotate: Simplify header dotted line sizing

No need to use strlen, etc to figure that out, just use the return from
printf(), it will tell how wide the following line needs to be.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Link: http://lkml.kernel.org/r/20160630082955.GA30921@twins.programming.kicks-ass.net


[ split from a larger patch ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>