issue with previous declaration of 'int sysTickHook()' with 'C++' linkage

The code of my DCF77 scope dcf77/DCF77_Scope.ino at master · udoklein/dcf77 · GitHub compiles fine with Arduino 1.6.0 but gives the following error with Arduino 1.6.7:

DCF77_Scope:256: error: previous declaration of 'int sysTickHook()' with 'C++' linkage
     int sysTickHook(void) {
     ^
DCF77_Scope:256: error: conflicts with new declaration with 'C' linkage
     int sysTickHook(void) {
                         ^
exit status 1
previous declaration of 'int sysTickHook()' with 'C++' linkage

Since the code compiles fine with 1.6.0 I assume there was a change to the Arduino IDE or the sources that cause this issue. Has anyone a hint for me what might cause this issue and how I can fix it?

Strange enough the DCF77 library which uses the same hook still compiles fine with both versions of Arduino.