cmake : Link vulkan-shaders-gen with pthreads (#8835)

cmake : Link vulkan-shaders-gen with pthreads (#8835)

When using CMake to build with Vulkan support, compiling
vulkan-shaders-gen fails due to missing a CMakeLists.txt specification
to link vulkan-shaders-gen with the threading library, resulting in the
following error.

    [5/172] Linking CXX executable bin/vulkan-shaders-gen
    FAILED: bin/vulkan-shaders-gen
    : && /usr/bin/c++ ggml/src/vulkan-shaders/CMakeFiles/vulkan-shaders-gen.dir/vulkan-shaders-gen.cpp.o -o bin/vulkan-shaders-gen   && :
    ld: error: undefined symbol: pthread_create
    >>> referenced by vulkan-shaders-gen.cpp
    >>>               ggml/src/vulkan-shaders/CMakeFiles/vulkan-shaders-gen.dir/vulkan-shaders-gen.cpp.o:(std::__1::__libcpp_thread_create[abi:se180100](pthread**,
    >>>               void* (*)(void*), void*))
    c++: error: linker command failed with exit code 1 (use -v to see invocation)
    [6/172] Generating build details from Git
    -- Found Git: /usr/local/bin/git (found version "2.45.2")
    ninja: build stopped: subcommand failed.

Add the CMakeLists.txt specification to link vulkan-shaders-gen with the
threading library and fix the above error.

Fixes #8834