graphics/mesa-gallium-vdpau: Remove libvdpau from LIB_DEPENDS

This port provides a lib for libvdpau to use, not the other way
around so add it to BUILD_DEPENDS

Differential Revision: https://reviews.freebsd.org/D44830
PR: 278324
Reported by: autodep
Reviewed by: bapt
Sponsored by: Beckhoff Automation GmbH & Co. KG