snippet: Updated all snippet tags

Ensure all src C++ headers which have corresponding C++ examples have
doxygen tags which point to those examples.  Some were missing, some
were invalid, some needed to be updated to match the new example names.

Signed-off-by: Noel Eck <noel.eck@intel.com>

src/speaker/speaker.hpp

@@ -65,6 +65,7 @@ namespace upm {
      *
      * @image html speaker.jpg
      * @snippet speaker.cxx Interesting
+     * @snippet speaker-pwm.cxx Interesting
      */
     class Speaker {
     public: