package/ffmpeg: add optional support for alsa-lib

ffmpeg has optional support for alsa as input and/or output device:
http://git.videolan.org/?p=ffmpeg.git;a=blob;f=configure;h=23823e3b7012d847b614bd43316fb614676bedb2;hb=refs/heads/release/3.3#l2987

Problem was found while fixing
http://autobuild.buildroot.net/results/7ba/7ba485532fcab74928246a8f95dba7e5eea9d4a5/

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/ffmpeg/ffmpeg.mk

@@ -157,12 +157,18 @@ endif
 
 ifeq ($(BR2_PACKAGE_FFMPEG_INDEVS),y)
 FFMPEG_CONF_OPTS += --enable-indevs
+ifeq ($(BR2_PACKAGE_ALSA_LIB),y)
+FFMPEG_DEPENDENCIES += alsa-lib
+endif
 else
 FFMPEG_CONF_OPTS += --disable-indevs
 endif
 
 ifeq ($(BR2_PACKAGE_FFMPEG_OUTDEVS),y)
 FFMPEG_CONF_OPTS += --enable-outdevs
+ifeq ($(BR2_PACKAGE_ALSA_LIB),y)
+FFMPEG_DEPENDENCIES += alsa-lib
+endif
 else
 FFMPEG_CONF_OPTS += --disable-outdevs
 endif