net/liferea: unbreak build

liferea tries to initialize it's graphics backend even when being run
for gobject introspection only - and that causes webkit to crash or
hang when there is no X server or similar available (e.g. during package
build in poudriere). This environment variable makes webkit survive
the package build.

PR: 243680
Submitted by: v1ne2go
Approved by: portmgr blanket (trivial and tested build fix)