the `-p servo` (formerly libservo) lib target was busted for anyone
genuinely consuming it as a library, but we didn’t catch it because the
CI job designed to check it built `-p servo` with --all-targets, which
unlike library consumers pulls in [dev-dependencies].
this patch fixes the compile error, and fixes the CI check to build `-p
servo` in a more realistic configuration.
Testing: tested in CI, and locally with `cargo build -p servo --locked`
Fixes: #43168
Signed-off-by: delan azabani <dazabani@igalia.com>