perf tools: Fix build break on powerpc

Commit 531d2410 ("perf tools: Do not include stringify.h from the
kernel sources") seems to have accidentially removed the inclusion of
"util/header.h" from "arch/powerpc/util/header.c".

"util/header.h" provides the prototype for get_cpuid() and is needed to
build perf on Powerpc:

	arch/powerpc/util/header.c:17:1: error: no previous prototype for 'get_cpuid' [-Werror=missing-prototypes]

Reported-by: Michael Ellerman <mpe@ellerman.id.au>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Fixes: 531d2410 ("perf tools: Do not include stringify.h from the kernel sources")
Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
[ Included "util.h" too, to get the scnprintf() prototype ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>