ffmpeg: add libswresample check

Adds libswresample to the package check. Namely, we don't want to use
libavresample, which has a different API. The latter exists solely for
compatibility with Libav.

Change-Id: I717975c696e2de2787be98cac07862215095b667

contrib/src/ffmpeg/rules.mak

@@ -3,7 +3,7 @@ FFMPEG_URL := https://git.ffmpeg.org/gitweb/ffmpeg.git/snapshot/$(FFMPEG_HASH).t
 
 PKGS+=ffmpeg
 
-ifeq ($(call need_pkg,"libavutil >= 55.75.100 libavcodec >= 57.106.101 libavformat >= 57.82.100 libavdevice >= 57.8.101 libavfilter >= 6.105.100 libswscale >= 4.7.103"),)
+ifeq ($(call need_pkg,"libavutil >= 55.75.100 libavcodec >= 57.106.101 libavformat >= 57.82.100 libavdevice >= 57.8.101 libavfilter >= 6.105.100 libswscale >= 4.7.103 libswresample >= 2.9.100"),)
 PKGS_FOUND += ffmpeg
 endif