api: make frame copying const for source frame

Don't bump version, as API is not broken. Breaks ABI, but we do not set
the SONAME tag, meaning we don't have an ABI version.

Change-Id: I5bc436f00e20eca7a789b0d8f5863d6622f82d0f
Reviewed-by: Sebastien Blin <sebastien.blin@savoirfairelinux.com>