opencl: bugfix increment ref_count in ggml_backend_opencl_init() (llama/26162)

opencl: bugfix increment ref_count in ggml_backend_opencl_init() (llama/26162)

Incrementing `ref_count` at the beginning is important later
in the `free()` method of the `ggml_backend_opencl_context` at program end.
If we do not increment the `ref_count`, the result would be -1 here,
and consequently, the profiling data would not be flushed and written.
( #ifdef GGML_OPENCL_PROFILING )