Add -ota to output field for version 0.2.2 of Privileged Extension (OTA)

As already mentioned in privileged-extension#24 (closed), the build for version 0.2.2 of the ota version of the privileged extension is currently failing, presumably because the commit tag is 0.2.2-ota, so the build.gradle outputs FDroidPrivilegedExtensionFromBinaries-0.2.2-ota.zip, but the metadata expects the output to be FDroidPrivilegedExtensionFromBinaries-0.2.2.zip.

This commit adds the missing -ota to the output field. I'm not sure if that is the right way to fix this but I'm looking forward to hearing your feedback on this.

(Note: I haven't actually tested the build.)