ffmpeg: Add LICENSE_FLAGS_ACCEPTED with commercial license

Add LICENSE_FLAGS_ACCEPTED to fix gstreamer1.0-libavi build, since this
recipe uses ffmpeg and the LICENSE_FLAGS_ACCEPTED is required.

Signed-off-by: Hiago De Franco <hiago.franco@toradex.com>

recipes-multimedia/ffmpeg/ffmpeg_4.4.1.bb

@@ -183,3 +183,5 @@ INSANE_SKIP:${MLPREFIX}libpostproc = "textrel"
 # Downgrade for NXP BSP
 DEFAULT_PREFERENCE = "-1"
 COMPATIBLE_MACHINE = "(imx-nxp-bsp)"
+
+LICENSE_FLAGS_ACCEPTED += " commercial "