fmt: fix import selective with interface implements (fix formatting part of #22200) (#22209)

Author: yuyi98

vlib/v/fmt/struct.v

@@ -38,6 +38,7 @@ pub fn (mut f Fmt) struct_decl(node ast.StructDecl, is_anon bool) {
 			if i < node.implements_types.len - 1 {
 				f.write(', ')
 			}
+			f.mark_types_import_as_used(t)
 		}
 	}
 	// Calculate the alignments first

vlib/v/fmt/tests/import_selective_with_implements_keep.vv

@@ -0,0 +1,10 @@
+import v.ast.walker { Visitor }
+import v.ast { Node }
+
+struct Walker implements Visitor {
+	aaa string
+}
+
+fn (mut n Walker) visit(node &Node) ! {
+	panic('not implemented')
+}