libqti-perfd-client: Fix function signatures and return values

[Flamefire]

• Use correct function signatures
• Remove namespace (not required due to extern "C"
• Put extern C into scope
• Return non-negative dummy values so callers don't see this as failed

Idea from: https://review.lineageos.org/c/LineageOS/android_device_xiaomi_sdm845-common/+/310583

Note: I'm quite sure the release part is wrong. It rather looks like an error code which should be returned, not the handle. The only place I could find this to be used checks against "-1". But can't tell for sure.

Closes https://github.com/whatawurst/android_device_sony_lilac/issues/33

[derfelot]

Thanks. As long as there is no errors in the logs, I think we should be good :)