add workspace root as dependency

Author: Lucas Pluvinage

dune

@@ -41,7 +41,8 @@
  (deps
   %{lib:gmp:libgmp.a}
   %{lib:gmp:libgmp.so}
-  %{lib:gmp:gmp.h})
+  %{lib:gmp:gmp.h}
+  %{workspace_root})
  (action
   (with-stdout-to
    %{target}