libs: Add forward declaration in C++ wrapper, required for MSVC

Props Jean Ghali.

Author: terryburton

libs/bindings/cpp/postscriptbarcode.hpp

@@ -73,6 +73,8 @@
 
 namespace bwipp {
 
+class BWIPP;
+
 /**
  * @brief Initialisation options for BWIPP construction.
  */