protection: Add a new definition for unspecified system protection

protection: Add a new definition for unspecified system protection

In some cases the system protection ID is not present in the contents
or in their metadata.
This define is used to set the value of the "system_id" field in GstProtectionEvent,
with this value, the application will use an external information to choose which
protection system to use.

Example: The matroskademux uses this value in the case of encrypted WebM,
the application will choose the appropriate protection system based on the information
received through EME API.

https://bugzilla.gnome.org/show_bug.cgi?id=797231