graphics/mesa-gallium-va: Remove libva from LIB_DEPENDS

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

Differential Revision: https://reviews.freebsd.org/D44829
PR: 278324
Reported by: autodep
Reviewed by: bapt
Sponsored by: Beckhoff Automation GmbH & Co. KG