Add missing libgbm1 dependency to GitLab CI section

Omitted in 0180a88,
despite prior 5c839f5.
See also puppeteer#290

Author: psibre

docs/troubleshooting.md

@@ -483,7 +483,7 @@ launch headless Chrome in your docker env:
 before_script:
   - apt-get update
   - apt-get install -yq gconf-service libasound2 libatk1.0-0 libc6 libcairo2
-    libcups2 libdbus-1-3 libexpat1 libfontconfig1 libgcc1 libgconf-2-4
+    libcups2 libdbus-1-3 libexpat1 libfontconfig1 libgbm1 libgcc1 libgconf-2-4
     libgdk-pixbuf2.0-0 libglib2.0-0 libgtk-3-0 libnspr4 libpango-1.0-0
     libpangocairo-1.0-0 libstdc++6 libx11-6 libx11-xcb1 libxcb1 libxcomposite1
     libxcursor1 libxdamage1 libxext6 libxfixes3 libxi6 libxrandr2 libxrender1