vboot_reference: change position of 'const'

BUG=chromium:230465
BRANCH=none
TEST=USE="chrome_internal"  CFLAGS="-clang -print-cmdline"
       CXXFLAGS="-clang -print-cmdline" emerge-x86-alex vboot_reference
       passes.

Change-Id: I42466ac27b1ad7f473fc9400461b553646c04617
Reviewed-on: https://gerrit.chromium.org/gerrit/47883
Reviewed-by: Bill Richardson <wfrichar@chromium.org>
Commit-Queue: Yunlian Jiang <yunlian@chromium.org>
Tested-by: Yunlian Jiang <yunlian@chromium.org>
1 file changed