libgphoto2 2.5.21 release

ptp2:
* Nikon Z series support added
* New Canon Powershot SX / GX also support capture, ALPHA support added
* Canon EOS bugfixes
* imported IDs from libmtp
* Added:
  * Nikon Z6, Z7
  * Canon Powershot SX720HS, G5X
  * Canon IXUS 185
