d/control: Move libotr5 to libotr5t64 for bin:thunderbird

Due the time64 transition the name for the name for the depending
Off-the-Record Messaging library has shifted from libotr5 to libotr5t64.

Unfortunately we need to work with the manually added dependency for the
thunderbird binary package. This library is linked at build time so also
not being detected by dpkg-shlibsdeps but later loaded dynamically if
user are using the chat modul.

See 73c48d45 for more details.

Closes: #1069337