lintian: ignore non multiarch install folder for thunderbird.pc

The pkg-config file thunderbird.pc isn't installed into
/u/l/$multiarch/pkg-config and this makes it impossible to use this in
a multiarchitecture purpose.
But once the next Thunderbird ESR version is starting the package
thunderbird-dev will go away. Also currently only two other packages are
know which depend on thunderbird-dev. These are
calendar-exchange-provider and sogo-connector and both resulting binary
packages won't work with Thunderbird >= 57 as Thunderbird uses
webextension as interface for plugins then.