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

Author: daym

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

@@ -7433,9 +7433,13 @@ static void ggml_vk_cleanup(ggml_backend_vk_context * ctx) {
 }
 
 static int ggml_vk_get_device_count() {
-    ggml_vk_instance_init();
-
-    return vk_instance.device_indices.size();
+    try {
+        ggml_vk_instance_init();
+        return vk_instance.device_indices.size();
+    } catch (const vk::SystemError& e) {
+        std::cerr << "ggml_vulkan: Error: System error " << e.what() << std::endl;
+        return 0;
+    }
 }
 
 static void ggml_vk_get_device_description(int device, char * description, size_t description_size) {