Event log had inconcistent support for increacing capabilies
number, as header were not inserted in capability buffer. It
resulted in a ghc-events crash (see #9384). This commit
fixes this issue by inserting required header when number
of capabilies grows.

Reviewers: simonmar, Mikolaj, trofi, austin

Reviewed By: Mikolaj, trofi, austin

Subscribers: carter, thomie

Differential Revision: https://phabricator.haskell.org/D592

GHC Trac Issues: #9384