conversation-web-view: de-globalize a variable

Implicitly declared variables in JS are global by default.

Fixes LGTM warning: "Variable header is used like a local variable, but
is missing a declaration."

Signed-off-by: Konstantin Kharlamov <Hi-Angel@yandex.ru>
5 jobs for <span class="ref-name">fix-lgtm-warnings</span> in 47 minutes and 39 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #251980
fedora

00:27:28

passed #251981
ubuntu

00:06:26

 
  Test
passed #251982
flatpak

00:19:17

 
  Deploy
passed #251983
review

00:00:34

failed #251984
allowed to fail manual
stop_review

00:00:17

 
Name Stage Failure
failed
stop_review Deploy
Using Docker executor with image fedora:29 ...
Pulling docker image fedora:29 ...
Using docker image sha256:d09302f77cfcc3e867829d80ff47f9e7738ffef69730d54ec44341a9fb1d359b for fedora:29 ...
Running on runner-d836cf30-project-6580-concurrent-0 via gcc150.bak.milne.osuosl.org...
Cloning repository...
Cloning into '/builds/Hi-Angel/geary'...
Checking out 585c7ba5 as fix-lgtm-warnings...
fatal: reference is not a tree: 585c7ba5bceb3252d77a78a4148b0aef8e89a340
ERROR: Job failed: exit code 1