Backport Vulkan-Hpp include fix

From https://github.com/KhronosGroup/Vulkan-Hpp/commit/d637c0e993a8a7a99c46018b48392c6e14fc7a6d

This should fix mac-swangle-chromium-x64:
https://ci.chromium.org/ui/p/chromium/builders/ci/mac-swangle-chromium-x64/108551/overview

Fixed: angleproject:552161106
Change-Id: I0679a4a52445ca55370c6e484ded71d8a55cd450
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/77788
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Tested-by: Antonio Maiorano <amaiorano@google.com>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>
Reviewed-by: Geoff Lang <geofflang@google.com>

include/vulkan/vulkan.hpp

diff --git a/include/vulkan/vulkan.hpp b/include/vulkan/vulkan.hpp
index fed6397..7a8755a 100644
--- a/include/vulkan/vulkan.hpp
+++ b/include/vulkan/vulkan.hpp
@@ -18,6 +18,9 @@
 #  include <string.h>  // strnlen
 #  include <string>    // std::string
 #  include <utility>   // std::exchange
+#  if defined( __APPLE__ )
+#    include <cstdlib>  // std::getenv
+#  endif
 #  if 17 <= VULKAN_HPP_CPP_VERSION
 #    include <string_view>
 #  endif