Default arguments should only be specified in function prototypes, not in the function definition. This caused compilation of the library to fail for Arduino AVR Boards 1.6.11 or earlier. It does not cause compilation to fail for Arduino AVR Boards 1.6.12 and newer only because the -fpermissive compiler flag was added, which downgrades this to an error. It has been stated that flag may be removed in the future:
https://github.com/arduino/ArduinoCore-avr/commit/06868f4cd39a9dba82b5014d1208feb1e1f0d750
So this change is necessary for both backwards and forwards compatibility. Even with current versions it fixes a warning, which is always wise to do.
Previously the library relied on the Arduino IDE doing the automatic insertion of #include <Arduino.h> in the sketch before #include <DDS.h>. In some IDE versions that does not happen and there is no guarantee of that behavior in future IDE versions thus for backwards and forwards compatibility it's best to simply add an #include directive to DDS.h.
per1234