include base/stringprintf.h directly

This code uses StringPrintf, but doesn't include the header.  Older
versions of libbase would implicitly include it, but newer ones do
not.  So include it ourselves to fix building with newer versions.

BUG=chromium-os:25872
TEST=`emerge-x86-alex metrics` works (with newer and older libbase)

Change-Id: I7859f67767958f70853a40e8df4a33a64b36d010
Reviewed-on: https://gerrit.chromium.org/gerrit/17487


Reviewed-by: Darin Petkov <petkov@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Commit-Ready: Mike Frysinger <vapier@chromium.org>