Declare explicit specializations

Resolves duplicate symbol build error on MinGW.

src/pv/pvData.h

@@ -436,6 +436,16 @@ private:
     friend class PVDataCreate;
 };
 
+/**
+ * @brief Some explicit specializations exist (defined in PVScalar.cpp)
+ */
+template<>
+    std::ostream& PVScalarValue<int8>::dumpValue(std::ostream& o) const;
+template<>
+    std::ostream& PVScalarValue<uint8>::dumpValue(std::ostream& o) const;
+template<>
+    std::ostream& PVScalarValue<boolean>::dumpValue(std::ostream& o) const;
+
 /**
  * typedefs for the various possible scalar types.
  */