Rutabaga: don't always build virglrenderer
Rutabaga: don't always build virglrenderer

If the virglrenderer feature isn't enabled, skip building virglrenderer.
This fixes cross-compiling for architectures that don't have libdrm
ready.

Change-Id: I397864ea6104896e1d12a838834d7d0b09a485be
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3221693
Commit-Queue: Dennis Kempin <cd09796fb571bec2782819dbfd33307f65b1c778@google.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Dennis Kempin <cd09796fb571bec2782819dbfd33307f65b1c778@google.com>

google/crosvm,google/crosvm,google/crosvm,google/crosvm