rhi: Guard QRhiVulkan more in qrhi_platform.h

Replicate qvulkaninstance.h's logic. The idea being to allow
to compile code that uses QRhi (so includes qrhi.h) with a pre-built
Qt that was built with Vulkan support, but there is no Vulkan SDK
(and so vulkan.h) on the developer's system. Normally this is not
something we care about, e.g. compiling an application using
QVulkanInstance or any other Vulkan functionality implies one has
to have the SDK installed or at least the headers available in one
way or another.

However, including qrhi.h (and so qrhi_platform.h) happens in some
cases in applications that do not themselves initialize a QRhi,
i.e. they do not care about the QRhiVulkan* structs, because
they only ever use a QRhi retrieved e.g. from the QQuickWindow.
Design Studio is one example. To allow building DS against a Qt
that is Vulkan-enabled (e.g. the official Qt releases) but in
an environment that has no Vulkan headers, we skip the QRhiVulkan*
stuff when there is no vulkan.h available.

Pick-to: 6.6
Change-Id: Ic81250e6c90939a38f79cab1438fbb9f483f56df
Reviewed-by: Miikka Heikkinen <miikka.heikkinen@qt.io>

src/gui/rhi/qrhi_platform.h

@@ -66,7 +66,7 @@ struct Q_GUI_EXPORT QRhiGles2NativeHandles : public QRhiNativeHandles
 
 #endif // opengl/qdoc
 
-#if QT_CONFIG(vulkan) || defined(Q_QDOC)
+#if (QT_CONFIG(vulkan) && __has_include(<vulkan/vulkan.h>)) || defined(Q_QDOC)
 
 struct Q_GUI_EXPORT QRhiVulkanInitParams : public QRhiInitParams
 {