debian/control: replace priority "extra" with "optional"

This fixes the following lintian warning:

W: libam7xxx-dev: priority-extra-is-replaced-by-priority-optional
N:
N:    Since Debian Policy version 4.0.1, the priority extra has been
N:    deprecated.
N:
N:    Please update debian/control and replace all instances of Priority:
N:    extra with Priority: optional.
N:
N:    Refer to Debian Policy Manual section 2.5 (Priorities) for details.
N:
N:    Severity: minor, Certainty: certain
N:
N:    Check: fields, Type: binary, udeb, source
N: