dbus/values_util_unittest.cc: use sqrt from cmath

This should fix the following error on an ASan Linux builder:

../../dbus/values_util_unittest.cc:360:15: error: use of undeclared identifier 'sqrt'
    keys[i] = sqrt(values[i]);
              ^

BUG=none
[email protected]
TBR=satorux

Review URL: https://codereview.chromium.org/1083323005

Cr-Commit-Position: refs/heads/master@{#326882}
1 file changed