perf annotate: Add number of samples to the header

Staring at annotations of large functions is useless if there's only a
few samples in them. Report the number of samples in the header to make
this easier to determine.

Committer note:

The change amounts to:

  - Percent | Source code & Disassembly of perf-vdso.so for cycles:u
  ------------------------------------------------------------------
  + Percent | Source code & Disassembly of perf-vdso.so for cycles:u (3278 samples)
  +--------------------------------------------------------------------------------

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>