cabal: add default-language
cabal: add default-language

can't do package uploads without this:

Uploading /home/tycho/packages/dist-newstyle/sdist/xcffib-0.10.0.tar.gz...
Error uploading /home/tycho/packages/dist-newstyle/sdist/xcffib-0.10.0.tar.gz:
http code 400
Error: Invalid package

Packages using 'cabal-version: >= 1.10' must specify the 'default-language'
field for each component (e.g. Haskell98 or Haskell2010). If a component uses
different languages in different modules then list the other ones in the
'other-languages' field.

Signed-off-by: Tycho Andersen <6b523322678240882acbb4da5312a12d8cd0082d@tycho.ws>

tych0/xcffib