[fidlc] Filter out zx when checking unused libs

Without filtering, the output was confusing:

    $ cat useszx.fidl
    library usesszx;
    using zx;
    struct Foo { zx.status status; };
    $ cat doesnotusezx.fidl
    library doesnotusezx;
    $ fidlc --files useszx.fidl --files doesnotusezx.fidl
    warning: Unused libraries provided via --files: usesszx, zx

Change-Id: Ia63ad15c3f047839c7c07218cddf627bafd062e6