* [c_api] minor tweak on Index_c header imports

* [c_api] Multi GPU constructs

- Add constructor & destructor for `FaissGpuMultipleClonerOptions`
- Add function `faiss_index_cpu_to_gpu_multiple`
- Add function `faiss_index_cpu_to_gpu_multiple_with_options`

* [c_api] fix doc comments in GpuClonerOptions

* [c_api] Improve cpu_to_gpu_multiple prototype

* [c_api] Improve cpu_to_gpu_multiple prototype

* [c_api] Add dtor definition for GpuMultipleClonerOptions