testing/thunderbird: fix FTBFS

error: casts cannot be followed by a field access
   --> servo/components/style/stylesheets/viewport_rule.rs:314:39
    |
314 |                     declaration!($min(value: shorthand.0, important: important)),
    |                                       ^^^^^^^^^^^^^^^^
...
323 |             "width" => ok!(shorthand -> [MinWidth, MaxWidth]),
    |                        -------------------------------------- in this macro invocation
    |
    = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
help: try surrounding the expression in parentheses
    |
314 |                     declaration!($min((value: shorthand).0, important: important)),
7 jobs for !7188 with testing/thunderbird in 21 seconds (queued for 1 second)
detached
Status Job ID Name Coverage
  Lint
failed #101195
docker-alpine x86_64 allowed to fail
lint

00:00:20

 
  Build
canceled #101200
docker-alpine ci-build aarch64
build-aarch64

canceled #101201
docker-alpine ci-build armv7
build-armv7

canceled #101199
docker-alpine ci-build ppc64le
build-ppc64le

canceled #101198
docker-alpine ci-build s390x
build-s390x

00:00:01

canceled #101197
docker-alpine ci-build x86
build-x86

00:00:00

canceled #101196
docker-alpine ci-build x86_64
build-x86_64

 
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failed
lint Lint
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