Change gcc-multilib to recommends in deb-packages

The gcc-multilib package is not required for dmd to work on 64-bit
systems when 32-bit software is not going to be built. Changing from
'Depends' to 'Recommends' still suggest a strong connection but it will
be possible to choose not to install gcc-multilib.

According to the Debian Policy Manual 'Recommends' still states the packages should be installed together during normal conditions. In my case I ran into problems and though a more relaxed dependency chain would be beneficial.
https://www.debian.org/doc/debian-policy/ch-relationships.html#s-binarydeps