[graphite] Define freeGpuData() in VulkanTexture class

A prior CL (https://skia-review.googlesource.com/c/skia/+/634499) included the VulkanTexture class for the first time, exposing an error within some Debian bots where VulkanTexture failed to define an override to the purely virtual Resource::freeGpuData() method.

Change-Id: I2307afc80d0b8e0abd5a4ab4c61429e48e2cc902
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/638980
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Nicolette Prevost <nicolettep@google.com>