add slices dep (#572)

Author: lucix-aws

codegen/smithy-go-codegen/src/main/java/software/amazon/smithy/go/codegen/SmithyGoDependency.java

@@ -49,6 +49,7 @@ public final class SmithyGoDependency {
     public static final GoDependency OS = stdlib("os");
     public static final GoDependency PATH_FILEPATH = stdlib("path/filepath");
     public static final GoDependency REFLECT = stdlib("reflect");
+    public static final GoDependency SLICES = stdlib("slices");
 
     public static final GoDependency SMITHY = smithy(null, "smithy");
     public static final GoDependency SMITHY_TRANSPORT = smithy("transport", "smithytransport");