Add default-language field

hindent.cabal

@@ -29,6 +29,7 @@ source-repository head
 library
   hs-source-dirs:    src/
   ghc-options:       -Wall -O2
+  default-language:  Haskell2010
   exposed-modules:   HIndent
                      HIndent.Types
                      HIndent.Pretty
@@ -52,6 +53,7 @@ library
 executable hindent
   hs-source-dirs:    src/main
   ghc-options:       -Wall -O2
+  default-language:  Haskell2010
   main-is:           Main.hs
   other-modules: Path.Find
   build-depends:     base >= 4 && < 5
@@ -74,6 +76,7 @@ executable hindent
 test-suite hindent-test
   type: exitcode-stdio-1.0
   hs-source-dirs: src/main/
+  default-language:  Haskell2010
   main-is: Test.hs
   other-modules: Markdone
   build-depends:     base >= 4 && <5
@@ -93,6 +96,7 @@ test-suite hindent-test
 benchmark hindent-bench
   type: exitcode-stdio-1.0
   hs-source-dirs:    src/main
+  default-language:  Haskell2010
   ghc-options:       -Wall -O2 -rtsopts
   main-is:           Benchmark.hs
   other-modules: Markdone