fix Cargo toml

Author: weikengchen

Cargo.toml

@@ -60,6 +60,6 @@ debug = true
 
 [features]
 default = ["std", "parallel"]
-std = [ "ark-ff/std", "ark-ec/std", "ark-poly/std", "ark-std/std", "ark-serialize/std" ]
+std = [ "ark-ff/std", "ark-ec/std", "ark-poly/std", "ark-std/std", "ark-relations/std", "ark-serialize/std" ]
 print-trace = [ "bench-utils/print-trace" ]
 parallel = [ "std", "ark-ff/parallel", "ark-ec/parallel", "ark-poly/parallel", "ark-std/parallel", "rayon" ]