vulkan: Make Vulkan optional at runtime (#11493).

Author: daym

ggml/src/ggml-vulkan/ggml-vulkan.cpp

@@ -8305,8 +8305,13 @@ ggml_backend_reg_t ggml_backend_vk_reg() {
         /* .iface       = */ ggml_backend_vk_reg_i,
         /* .context     = */ nullptr,
     };
-
-    return ®
+    try {
+        ggml_vk_instance_init();
+        return ®
+    } catch (const vk::SystemError& e) {
+        VK_LOG_DEBUG("ggml_backend_vk_reg() -> Error: System error: " << e.what());
+        return nullptr;
+    }
 }
 
 // Extension availability