gnutls: Reorder ‘static’ and ‘const’ attributes

gcc prefers them this way round — the other way round is old-style.

https://bugzilla.gnome.org/show_bug.cgi?id=736809
gnutls: Fix a signed/unsigned integer comparison

https://bugzilla.gnome.org/show_bug.cgi?id=736809
gnutls: Add a missing enum value to a switch statement

And a missing default case. This introduces no functional changes.

https://bugzilla.gnome.org/show_bug.cgi?id=736809