avcodec/fft_template: use ff_thread_once on costable initialization

Make it thread-safe and avoid redundant initialization.

Reviewed-by: Rostislav Pehlivanov <atomnuker@gmail.com>
Reviewed-by: wm4 <nfxjfg@googlemail.com>
Signed-off-by: Muhammad Faiz <mfcc64@gmail.com>