Fix build with latest dune.

Author: xclerc

src/dune

@@ -8,5 +8,5 @@
  (deps (:first_dep ../gen/zstd_gen.exe)) (action (bash ./%{first_dep})))
 
 (rule (targets explicit_dependencies.ml explicit_dependencies.mli)
- (deps %{workspace_root}/bin/gen-explicit-dependencies.sh)
- (action (bash "%{deps} zstd_c")))
+ (mode fallback) (deps %{workspace_root}/bin/gen-explicit-dependencies.sh)
+ (action (bash "%{deps} zstd_c")))