gf.cabal: add missing c-sources for the Haskell binding to the C run-time library

gf.cabal

@@ -130,6 +130,7 @@ Library
     hs-source-dirs:      src/runtime/haskell-bind
     build-tools:         hsc2hs
     extra-libraries:     gu pgf
+    c-sources:           src/runtime/haskell-bind/utils.c
     cc-options:          -std=c99
 
 Executable gf