Backends: Vulkan: Fix building with VK_NO_PROTOTYPES. (#914, #5738).

Amend a588f00

backends/imgui_impl_vulkan.cpp

@@ -191,6 +191,7 @@ static bool g_FunctionsLoaded = true;
     IMGUI_VULKAN_FUNC_MAP_MACRO(vkDeviceWaitIdle) \
     IMGUI_VULKAN_FUNC_MAP_MACRO(vkFlushMappedMemoryRanges) \
     IMGUI_VULKAN_FUNC_MAP_MACRO(vkFreeCommandBuffers) \
+    IMGUI_VULKAN_FUNC_MAP_MACRO(vkFreeDescriptorSets) \
     IMGUI_VULKAN_FUNC_MAP_MACRO(vkFreeMemory) \
     IMGUI_VULKAN_FUNC_MAP_MACRO(vkGetBufferMemoryRequirements) \
     IMGUI_VULKAN_FUNC_MAP_MACRO(vkGetImageMemoryRequirements) \