xmonad.cabal: Specify default-language

Required for cabal-version >= 1.10

xmonad.cabal

@@ -80,6 +80,7 @@ library
                  , transformers          >= 0.3
                  , unix
   ghc-options:     -funbox-strict-fields -Wall -Wno-unused-do-bind
+  default-language: Haskell2010
 
   -- Keep this in sync with the oldest version in 'tested-with'
   if impl(ghc > 8.4.4)
@@ -92,6 +93,7 @@ executable xmonad
   main-is:       Main.hs
   build-depends: base, xmonad
   ghc-options:   -Wall -Wno-unused-do-bind
+  default-language: Haskell2010
 
   -- Keep this in sync with the oldest version in 'tested-with'
   if impl(ghc > 8.4.4)
@@ -126,6 +128,7 @@ test-suite properties
                 , X11
                 , containers
                 , xmonad
+  default-language: Haskell2010
 
   if flag(quickcheck-classes) && impl(ghc > 8.5)
     -- no quickcheck-classes in LTS-12