cmake: fix macOS build with -DGGML_BACKEND_DL=ON

[giladgd]

#17091 introduced an issue where when building with -DGGML_BACKEND_DL=ON on macOS you get this error:

c++: error: invalid argument '-current_version 0.9.4' only allowed with '-dynamiclib'
make[2]: *** [bin/libggml-blas.so] Error 1
make[1]: *** [ggml/src/ggml-blas/CMakeFiles/ggml-blas.dir/all] Error 2

It appears that building a module library with a version is not supported on macOS (CMake ticket), so this PR makes the build skip setting a version for backend module libraries on macOS.