mplayer should complete on m4b files

[Götz Waschk]

mplayer completion in /usr/share/zsh/functions/Completion/X/_mplayer allows completion on several media file types, e.g. mp3 or mkv. Please add m4b to this list, it is the same as m4a, but used for audio books.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: zsh-common 5.1.1-1ubuntu2
ProcVersionSignature: Ubuntu 4.10.0-42.46~16.04.1-generic 4.10.17
Uname: Linux 4.10.0-42-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.14
Architecture: amd64
CurrentDesktop: GNOME
Date: Thu Dec 28 20:48:34 2017
DistributionChannelDescriptor:
 # This is a distribution channel descriptor
 # For more information see http://wiki.ubuntu.com/DistributionChannelDescriptor
 canonical-oem-somerville-trusty-amd64-20140620-0
InstallationDate: Installed on 2015-06-19 (923 days ago)
InstallationMedia: Ubuntu 14.04 "Trusty" - Build amd64 LIVE Binary 20140620-04:25
PackageArchitecture: all
SourcePackage: zsh
UpgradeStatus: Upgraded to xenial on 2016-04-25 (612 days ago)