CI: need to install zstd.

Author: vtraag

appveyor.yml

@@ -51,6 +51,7 @@ before_build:
 install:
   - bash -lc "pacman --needed --noconfirm -Sy pacman-mirrors"
   - bash -lc "pacman --noconfirm -Sy"
+  - bash -lc "pacman --noconfirm -S zstd"
   - bash -lc "pacman --noconfirm -S autoconf automake bison flex libtool mingw-w64-x86_64-libtool mingw-w64-i686-libxml2 mingw-w64-x86_64-libxml2 zip"
   - bash -lc "pacman --noconfirm -Rs git" # Must be removed in order to use Windows git
   - python.exe -m pip install cibuildwheel==1.3.0