Fix: Pickle Label flag in TreePickler

Was forgotten before, which meant that labels were forgotten in inlined
code. The code was still correct, but a method would be created for each label.

Author: odersky

compiler/src/dotty/tools/dotc/core/tasty/TreePickler.scala

@@ -607,6 +607,7 @@ class TreePickler(pickler: TastyPickler) {
       if (flags is DefaultParameterized) writeByte(DEFAULTparameterized)
       if (flags is Stable) writeByte(STABLE)
       if ((flags is ParamAccessor) && sym.isSetter) writeByte(PARAMsetter)
+      if ((flags is Label)) writeByte(LABEL)
     } else {
       if (flags is Sealed) writeByte(SEALED)
       if (flags is Abstract) writeByte(ABSTRACT)