debian/control: bump Standards-Version to 4.1.3

This silences a lintian notice:

W: libam7xxx source: ancient-standards-version 3.9.6 (released 2014-09-17) (current is 4.1.3)
N:
N:    The source package refers to a Standards-Version that has been obsolete
N:    for more than two years. Please update your package to latest Policy and
N:    set this control field appropriately.
N:
N:    If the package is already compliant with the current standards, you
N:    don't have to re-upload the package just to adjust the Standards-Version
N:    control field. However, please remember to update this field next time
N:    you upload the package.
N:
N:    See /usr/share/doc/debian-policy/upgrading-checklist.txt.gz in the
N:    debian-policy package for a summary of changes in newer versions of
N:    Policy.
N:
N:    Refer to
N:    https://www.debian.org/doc/packaging-manuals/upgrading-checklist.txt for
N:    details.
N:
N:    Severity: normal, Certainty: certain
N:
N:    Check: standards-version, Type: source
N: