Resolve segfault in wsialloc_delete

A segfault can occur if wsialloc_delete is called but m_wsi_allocator's
ion field has not been allocated yet, as the function attempts to access
the ion properties.

This commit resolves the issue by changing m_wsi_allocator to an opaque
struct and removing the fd property, which was not being used.
wsialloc_new is then changed to return a pointer to the struct and
the swapchain then carries this pointer around, which will be
initialised to nullptr in the swapchain constructor.

Change-Id: I202e967cff4555babf0ddcd387275b74470b09d5
Signed-off-by: Dennis Tsiang <dennis.tsiang@arm.com>
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build_vulkan_wsi_layer

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