intel: add -Wmissing-declarations
intel: add -Wmissing-declarations

Useful when a function is removed from the public API but the driver is not
updated.

elongbug/Vulkan-LoaderAndValidationLayers,Radamanthe/VulkanSamples,elongbug/Vulkan-LoaderAndValidationLayers,critsec/Vulkan-LoaderAndValidationLayers,Radamanthe/VulkanSamples,critsec/Vulkan-LoaderAndValidationLayers,sashinde/VulkanTools,Radamanthe/VulkanSamples,KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers,sashinde/VulkanTools,sashinde/VulkanTools,Radamanthe/VulkanSamples,sashinde/VulkanTools,elongbug/Vulkan-LoaderAndValidationLayers,Radamanthe/VulkanSamples,KhronosGroup/Vulkan-LoaderAndValidationLayers,elongbug/Vulkan-LoaderAndValidationLayers,critsec/Vulkan-LoaderAndValidationLayers,Radamanthe/VulkanSamples,critsec/Vulkan-LoaderAndValidationLayers