virtio-gpu-3d: Define VIRTIO_GPU_CAPSET_VIRGL2 elsewhere

Commit 5643cc94 ("virtio-gpu-3d: add support for second capability
set (v4)") updated virtio_gpu.h with a define that does not yet(?)
exist upstream resulting in build breakage every time Linux headers
are updated via the standard update script.  Conditionally define this
within QEMU code instead to avoid future breakage.

Cc: Dave Airlie <airlied@redhat.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Fixes: 5643cc94 ("virtio-gpu-3d: add support for second capability set (v4)")
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20180525132755.21839-2-peter.maydell@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>