render/vulkan: Fix clang compiler error in 32 bit ARM Android

This adds proper calling convention attribute for callbacks used in vulkan
and fixes the following compiler error.

../src/render/vulkan/vulkan.c:175:22: error: incompatible function pointer types
initializing 'PFN_vkDebugUtilsMessengerCallbackEXT' (aka 'unsigned int (*)
(enum VkDebugUtilsMessageSeverityFlagBitsEXT, unsigned int, const struct
VkDebugUtilsMessengerCallbackDataEXT *, void *) __attribute__((pcs("aapcs-vfp")))')
with an expression of type 'VkBool32 (*)(VkDebugUtilsMessageSeverityFlagBitsEXT,
VkDebugUtilsMessageTypeFlagsEXT, const VkDebugUtilsMessengerCallbackDataEXT *, void *)'
(aka 'unsigned int (*)(enum VkDebugUtilsMessageSeverityFlagBitsEXT, unsigned int,
const struct VkDebugUtilsMessengerCallbackDataEXT *, void *)')
[-Wincompatible-function-pointer-types]
    .pfnUserCallback = &debug_callback,
                       ^~~~~~~~~~~~~~~

render/vulkan/vulkan.c

@@ -31,7 +31,7 @@ static bool check_extension(const VkExtensionProperties *avail,
 	return false;
 }
 
-static VkBool32 debug_callback(VkDebugUtilsMessageSeverityFlagBitsEXT severity,
+static VKAPI_ATTR VkBool32 debug_callback(VkDebugUtilsMessageSeverityFlagBitsEXT severity,
 		VkDebugUtilsMessageTypeFlagsEXT type,
 		const VkDebugUtilsMessengerCallbackDataEXT *debug_data,
 		void *data) {