perf: Fix implicit declaration of getline in util.c

getline() is considered as undeclared in util/util.c because
it includes string.h, that in turn includes stdio.h, without
having defined _GNU_SOURCE.

But util.c also includes util.h that handles the _GNU_SOURCE and
all the needed inclusions already. Let's include only util.h
and sys/mman.h which is the only one header not handled by
util.h

This fixes the following build error:

 util/util.c: In function 'slow_copyfile':
 util/util.c:49: erreur: implicit declaration of function
 'getline' util/util.c:49: erreur: nested extern declaration of 'getline'

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Paul Mackerras <paulus@samba.org>
LKML-Reference: <1263648075-3858-1-git-send-regression-fweisbec@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>