meson/vdpau: add missing soversion

This mirrors what autotools does in src/gallium/state_trackers/vdpau/Makefile.am
and src/gallium/targets/vdpau/Makefile.am:

  VDPAU_MAJOR = 1
  VDPAU_MINOR = 0
  libvdpau_gallium_la_LDFLAGS = -version-number $(VDPAU_MAJOR):$(VDPAU_MINOR)

Reported-by: Igor Gnatenko <i.gnatenko.brain@gmail.com>
Fixes: 68076b87 "meson: build gallium vdpau state tracker"
Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
(cherry picked from commit 69e94403)