ffmpeg: use nasm patched-in debug-prefix-map option to restore reproducibility

Latest nasm requires this as it otherwise writes full paths into its output.

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>