Add missing package for AppVeyor CI Ubuntu build

Author: adamretter

appveyor.yml

@@ -19,7 +19,7 @@ stack: node 10
 install:
   - cmd: PowerShell.exe "Install-Product node $env:nodejs_version"
   - node --version && yarn --version && python --version
-  - sh: sudo apt-get --yes install libx11-dev libxkbfile-dev
+  - sh: sudo apt-get --yes install libx11-dev libxkbfile-dev rpm
   - cmd: yarn
   - sh: yarn