vulkan: Have a dummy format type if there is no Vulkan header

...at application build time.

Follow what is already done for some other Vk* types.

Supports 1df2cf6bad7207f16ddca17344cc1324e50f287e in qtdeclarative,
which puts now also VkFormat into a header. We need the function
declaration to compile without a Vulkan SDK (so without vulkan.h).

Fixes: QTBUG-104501
Change-Id: I09c87149a53a45f03c9f75b87baec4323db16b1d
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
(cherry picked from commit c0509ef586e939fbc60b52f80f2cf1b40e5bd9be)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/gui/vulkan/qvulkaninstance.h

@@ -36,6 +36,7 @@ typedef void* VkPhysicalDevice;
 typedef void* VkDevice;
 // enums
 typedef int VkResult;
+typedef int VkFormat;
 typedef int VkImageLayout;
 typedef int VkDebugReportFlagsEXT;
 typedef int VkDebugReportObjectTypeEXT;