perf tools: Put common histogram functions in their own file

Move histogram related functions into their own files (hist.c and
hist.h) and make use of them in builtin-annotate.c and
builtin-report.c.

Signed-off-by: John Kacur <jkacur@redhat.com>
Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <alpine.LFD.2.00.0909281531180.8316@localhost.localdomain>
Signed-off-by: Ingo Molnar <mingo@elte.hu>