perf: Swap inclusion order of util.h and string.h in util/string.c

Currently util/string.c includes headers in this order: string.h, util.h
But this causes a build error because __USE_GNU definition
is needed for strndup() definition:

	% make -j
	touch .perf.dev.null
	    CC util/string.o
	cc1: warnings being treated as errors
	util/string.c: In function ‘argv_split’:
	util/string.c:171: error: implicit declaration of function ‘strndup’
	util/string.c:171: error: incompatible implicit declaration of built-in function ‘strndup’

So this patch swaps the headers inclusion order.
util.h defines _GNU_SOURCE, and /usr/include/features.h defines
__USE_GNU as 1 if _GNU_SOURCE is defined.

Signed-off-by: Hitoshi Mitake <mitake@dcl.info.waseda.ac.jp>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
LKML-Reference: <1270368798-27232-1-git-send-email-mitake@dcl.info.waseda.ac.jp>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>