package/protobuf: Update site url

Package moved to github:
https://developers.google.com/protocol-buffers/docs/downloads
"Our old Google Code repositiory is: https://code.google.com/p/protobuf/

.
 We moved to github on Aug 26, 2014 [...]"

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>