Add cfg_attr to clippy warning

Author: kestred

diesel/src/pg/types/numeric.rs

@@ -94,7 +94,8 @@ mod bigdecimal {
 
             // Ensure that the decimal will always lie on a digit boundary
             for _ in 0..(4 - scale % 4) {
-                #[allow(assign_op_pattern)] // No `std::ops::MulAssign` impl for `BigInt`
+                // NOTE: No `std::ops::MulAssign` impl for `BigInt`
+                #[cfg_attr(feature = "cargo-clippy", allow(assign_op_pattern))]
                 integer = integer * 10;
             }
             let integer = integer.to_biguint().expect("integer is always positive");