This patch explicitly includes unicode/uvernum.h in the regular
expression parser.

It should be removed once we no longer need to check
`U_ICU_VERSION_MAJOR_NUM` during preprocessing, i.e. once Node.js
updates their ICU. This is an ongoing effort:
https://github.com/nodejs/node/pull/16876

BUG=v8:4743

Change-Id: I3cd9447b481249a9035d9fb00745057da8809c58
Reviewed-on: https://chromium-review.googlesource.com/758407Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Mathias Bynens <mathias@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49253}