vulkan: Make Vulkan optional at runtime (#11493). (#11494)

Co-authored-by: Jeff Bolz <jbolz@nvidia.com>

ggml/include/ggml-vulkan.h

@@ -10,8 +10,6 @@ extern "C" {
 #define GGML_VK_NAME "Vulkan"
 #define GGML_VK_MAX_DEVICES 16
 
-GGML_BACKEND_API void ggml_vk_instance_init(void);
-
 // backend API
 GGML_BACKEND_API ggml_backend_t ggml_backend_vk_init(size_t dev_num);