[sledge] Add missing build dep on env var

Summary:
Generating the help file runs sledge, which refers to the
SLEDGE_CONFIG environment variable. This dependency was missing from
the build rule.

Reviewed By: ngorogiannis

Differential Revision: D28907804

fbshipit-source-id: e20178c6e

sledge/dune

@@ -114,7 +114,10 @@
 
 (rule
  (targets sledge-help.txt)
- (deps tools/gen_help.sh cli/sledge_cli.exe)
+ (deps
+  tools/gen_help.sh
+  cli/sledge_cli.exe
+  (env_var SLEDGE_CONFIG))
  (action
   (with-stdout-to
    sledge-help.txt