vulkan: hide internal symbols to prevent duplicate-dlopen state destruction (llama/29139)

Since #28732 our internal symbols are exported. A duplicate copy dlopened and
dlclosed by ggml_backend_load_all() then interposes them, so its destructors
destroy the live vk_instance and later device queries hit the GGML_ASSERT on
vk_instance.device_indices. Hidden visibility exports only GGML_BACKEND_API,
as before #28732.

Fixes #29138

Assisted-by: henk:claude-fable-5

ggml/src/ggml-vulkan/CMakeLists.txt

@@ -69,6 +69,11 @@ if (Vulkan_FOUND)
                              ggml-vulkan-debug.cpp
                             )
 
+    # hide symbols, so dlclosed duplicate copies cannot interpose them
+    set_target_properties(ggml-vulkan PROPERTIES
+                          CXX_VISIBILITY_PRESET hidden
+                          VISIBILITY_INLINES_HIDDEN ON)
+
     set(VULKAN_SHADER_GEN_CMAKE_ARGS "")
 
     # Test all shader extensions