multimedia/mpv: track VDPAU dependency on X11

meson.build:1411:0: ERROR: Feature vdpau cannot be enabled: x11 was not found!

(or before 1d89ec25be62)

Checking for VDPAU acceleration : x11 not found
Checking for VDPAU with OpenGL/X11 : vdpau not found

PR: 267816
Reported by: amdmi3