misc/ncnn: unbreak new VULKAN feature after r539362

https://github.com/Tencent/ncnn/commit/17c445480fee
https://github.com/Tencent/ncnn/commit/eec5cf718015

CMake Warning at CMakeLists.txt:98 (message):

The submodules were not downloaded! NCNN_VULKAN_ONLINE_SPIRV will be turned
off.