meson: Update introspection version to 1.6
Update the introspection version to 1.6 as GUPnP is not compatible with previous versions, so you can't mix them...
Although the GUPnP-IGD API itself hasn't changed, I guess if it brings it an incompatible dependency, it's an API break anyway..
Fixes: #9 (closed)
Edited by Olivier Crête