NTUnionBuilder: Add missing value() function

Add missing function NTUnionBuilder::value() to allow the union type of
the NTUnion's value field to be specified.

src/ntunion.cpp

@@ -18,6 +18,12 @@ static NTFieldPtr ntField = NTField::get();
 namespace detail {
 
 
+NTUnionBuilder::shared_pointer NTUnionBuilder::value(UnionConstPtr unionType)
+{
+    valueType = unionType;
+    return shared_from_this();
+}
+
 StructureConstPtr NTUnionBuilder::createStructure()
 {
     FieldBuilderPtr builder =

src/pv/ntunion.h

@@ -40,6 +40,14 @@ namespace detail {
     public:
         POINTER_DEFINITIONS(NTUnionBuilder);
 
+        /**
+         * Specifies the union for the value field.
+         * If this is not called then a variant union is the default.
+         * @param unionType  the introspection object for the union value field
+         * @return this      instance of  NTUnionBuilder
+         */
+        shared_pointer value(epics::pvData::UnionConstPtr unionType);
+
         /**
          * Adds descriptor field to the NTUnion.
          * @return this instance of <b>NTUnionBuilder</b>.