Fix missing dependency error in example. Refs #252.

The counter example is missing the dependency on copilot-c99 in the
cabal file, resulting in an error during compilation.

This commit adds that dependency on copilot-c99 to the executable.

[ci skip]

Author: ivanperez-keera

copilot/copilot.cabal

@@ -162,6 +162,7 @@ executable counter
     hs-source-dirs:     examples
     build-depends:      base              >= 4.9  && < 5
                       , copilot
+                      , copilot-c99
     default-language:   Haskell2010
     if flag(examples)
       buildable: True