vulkan: Fix win/i386 calling convention

This fixes the following error when compiling with a modern
version of Clang for Windows/i386:

src/libavutil/hwcontext_vulkan.c:738:32: error: incompatible function pointer types initializing 'PFN_vkDebugUtilsMessengerCallbackEXT' (aka 'unsigned int (*)(enum VkDebugUtilsMessageSeverityFlagBitsEXT, unsigned int, const struct VkDebugUtilsMessengerCallbackDataEXT *, void *) __attribute__((stdcall))') 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 = vk_dbg_callback,
                               ^~~~~~~~~~~~~~~

Signed-off-by: Martin Storsjö <martin@martin.st>

libavutil/hwcontext_vulkan.c

@@ -401,10 +401,10 @@ static const char *vk_ret2str(VkResult res)
 #undef CASE
 }
 
-static VkBool32 vk_dbg_callback(VkDebugUtilsMessageSeverityFlagBitsEXT severity,
-                                VkDebugUtilsMessageTypeFlagsEXT messageType,
-                                const VkDebugUtilsMessengerCallbackDataEXT *data,
-                                void *priv)
+static VkBool32 VKAPI_CALL vk_dbg_callback(VkDebugUtilsMessageSeverityFlagBitsEXT severity,
+                                           VkDebugUtilsMessageTypeFlagsEXT messageType,
+                                           const VkDebugUtilsMessengerCallbackDataEXT *data,
+                                           void *priv)
 {
     int l;
     AVHWDeviceContext *ctx = priv;