fix return type of props_set_writable.
fix return type of props_set_writable.

OpenMusicKontrollers/chimaera.lv2,OpenMusicKontrollers/chimaera.lv2,OpenMusicKontrollers/orbit.lv2,OpenMusicKontrollers/eteroj.lv2,OpenMusicKontrollers/sherlock.lv2,OpenMusicKontrollers/espressivo.lv2,OpenMusicKontrollers/sherlock.lv2,OpenMusicKontrollers/orbit.lv2,OpenMusicKontrollers/synthpod,OpenMusicKontrollers/synthpod,OpenMusicKontrollers/synthpod,OpenMusicKontrollers/sherlock.lv2,OpenMusicKontrollers/synthpod,OpenMusicKontrollers/sherlock.lv2,OpenMusicKontrollers/eteroj.lv2,OpenMusicKontrollers/synthpod