ac/llvm: export ac_init_llvm_once in targets

ac/llvm: export ac_init_llvm_once in targets

If a program like mpv uses both radeon_dri.so (because --vo=gpu) and
radeonsi_drv_video.so (because --hwdec=vaapi) then LLVM will be inialized twice.

The commit exports the ac_init_llvm_once so there's only one instance of the
function.

See also 18b12bf5335 ("targets: export radeon winsys_create functions to silence LLVM warning")
which implemented this workaround initially.

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/1377
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Acked-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5648>