net/xrdp: Set MP3LAME option disabled by default

because audio/lame has no-pkg and xrdp doesn't work properly because of missing
library dependency on lame when xrdp is "pkg install"-ed.

[1] https://lists.freebsd.org/pipermail/freebsd-ports/2019-June/116470.html

Reported by: Janky Jay, III [1]