The Vulkan spec 1.1.97 says:
   "variablePointers specifies whether the implementation supports
    the SPIR-V VariablePointers capability. When this feature is
    not enabled, shader modules must not declare the
    VariablePointers capability."

As the SPIR-V feature is enabled, we should turn on the
extension feature as well.

All dEQP-VK.spirv_assembly.instruction.compute.variable_pointers.*
pass with the khronos internal repo. Note that a bunch of them
fails with the public repo, but it's expected as they violate the
specification.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>