fix a warning

langfilter/langfilter.cabal

@@ -11,7 +11,6 @@ common base
   ghc-options:
     -Wall -Wno-missing-signatures -Wno-missing-pattern-synonym-signatures
     -Wno-name-shadowing
-    -threaded -rtsopts -with-rtsopts=-N
   default-language: Haskell2010
   default-extensions:
     BlockArguments,
@@ -36,6 +35,10 @@ common base
     pretty-show ^>= 1.10,
     laantas-script
 
+common exe
+  ghc-options:
+    -threaded -rtsopts -with-rtsopts=-N
+
 library
   import: base
   hs-source-dirs: lib
@@ -48,7 +51,7 @@ library
     LangFilter
 
 executable langfilter
-  import: base
+  import: base, exe
   hs-source-dirs: main
   main-is: Main.hs
   build-depends: langfilter