Followup to https://codereview.chromium.org/2621983002/ so that
v8-version-string.h is includeable with varying include search paths.
This seems to match the pattern of other #includes in the include/
directory.

R=machenbach@chromium.org
BUG=chromium:581766

Review-Url: https://codereview.chromium.org/2634443002
Cr-Commit-Position: refs/heads/master@{#42337}