x11-servers/xwayland: unbreak build on 15.0 after Secure RPC DES authentication removal (+)

FreeDesktop upstream have dropped support for Secure RPC DES authentication
in xorg-servers more than year ago [1]

FreeBSD have it dropped in commit https://cgit.freebsd.org/src/commit/?id=7ac276298b72982189ac1a5b17461936dc00163e
As follow-up set meson build flag -Dsecure-rpc to false to avoid include a dead legacy code.

[1] https://gitlab.freedesktop.org/xorg/xserver/-/commit/71b207a2ebc1465c7d9ad9262f60930f6a1d42ee

Discussed with: ivy
Approved by: portmgr blanket