Silence GHC 8.4's "ignoring (possibly broken) abi-depends..." warning

The current version of this package does not work properly with GHC 8.4.
The reason is that the `ghc-pkg --package-db debian/tmp-db/ recache`
command outputs a warning about possibly broken abi-depends:

  ignoring (possibly broken) abi-depends field for packages

See also https://ghc.haskell.org/trac/ghc/ticket/14381.

This causes the output of the tmp_package_db() function to contain the
above string, and function package_id_to_virtual_package() fails with:

  /usr/share/haskell-devscripts/Dh_Haskell.sh: line 368: ignoring: command not found

Unfortunately, the above does not crash the build, it only leads to
empty values for the corresponding variables, hence all haskell packages
end up with a `Provides: libghc--dev--'.

Fix the above bug by ignoring the output of the `ghc-pkg recache`
command.