package/ffmpeg: fix typo "--enable-bsfs" -> "--enable-bsf"

package/ffmpeg: fix typo "--enable-bsfs" -> "--enable-bsf"

Fix ffmpeg configuration when BR2_PACKAGE_FFMPEG_BSFS is not default ("all"):

	Unknown option "--enable-bsfs=h264_metadata".
	See ./configure --help for available options.
	package/pkg-generic.mk:254: recipe for target '/root/buildroot/output/build/ffmpeg-4.2.2/.stamp_configured' failed

The option is named according to the scheme "--enable-SINGULAR=..." /
"--disable-PLURAL".

The typo is present since the release 2014.02:

https://git.buildroot.net/buildroot/commit/package/ffmpeg/ffmpeg.mk?id=62ab07ef769bd6504fe1db144aaac3fd45db9dad

Signed-off-by: Anatoly Borodin <anatoly.borodin@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>