package/ffmpeg: bump version to 3.4.6

Fixes a number of bugs discovered after the 3.4.5 release:
https://git.ffmpeg.org/gitweb/ffmpeg.git/shortlog/n3.4.6

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/ffmpeg/ffmpeg.hash

@@ -1,5 +1,5 @@
 # Locally calculated
-sha256 741cbd6394eaed370774ca4cc089eaafbc54d0824b9aa360d4b3b0cbcbc4a92c  ffmpeg-3.4.5.tar.xz
+sha256 3572279cb139d9e39dcfbc23edf438ff5311ec3fc5d0dcb3558e49591e5cb83e  ffmpeg-3.4.6.tar.xz
 sha256 8177f97513213526df2cf6184d8ff986c675afb514d4e68a404010521b880643  COPYING.GPLv2
 sha256 b634ab5640e258563c536e658cad87080553df6f34f62269a21d554844e58bfe  COPYING.LGPLv2.1
 sha256 73d99bc83313fff665b426d6672b4e0479102bc402fe22314ac9ce94a38aa5ff  LICENSE.md

package/ffmpeg/ffmpeg.mk

@@ -4,7 +4,7 @@
 #
 ################################################################################
 
-FFMPEG_VERSION = 3.4.5
+FFMPEG_VERSION = 3.4.6
 FFMPEG_SOURCE = ffmpeg-$(FFMPEG_VERSION).tar.xz
 FFMPEG_SITE = http://ffmpeg.org/releases
 FFMPEG_INSTALL_STAGING = YES