vulkan_core.h only includes 'core' Vulkan declarations, while vulkan.h
consists of vulkan_core.h plus platform-specific headers.

Only Window System Integration (WSI) and closely related source files
should have a need to include vulkan.h. Everything else, in particular
our back-end, should not require anything Vulkan platform-specific.

This avoids issues such as Xlib defining global macros which clash with
C++ type and variable names.

Bug b/124265819

Change-Id: I6c250f27aef39b98a2a8b677c169df060698f104
Reviewed-on: https://swiftshader-review.googlesource.com/c/24709Tested-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>