servo: Merge #19356 - update readme for ubuntu build (from tigercosmos:r1); r=jdm

<!-- Please describe your changes on the following line: -->

Fix Ubuntu build

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix  #19351 (github issue number if applicable).

Source-Repo: https://github.com/servo/servo
Source-Revision: 6009441f5ed52a1b723b386f6845b4fd4564c8f5

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : 0e0b8a69970252f270d22c91411b34cc4549f9c1

servo/README.md

@@ -56,8 +56,10 @@ sudo apt install git curl freeglut3-dev autoconf libx11-dev \
     libfreetype6-dev libgl1-mesa-dri libglib2.0-dev xorg-dev \
     gperf g++ build-essential cmake virtualenv python-pip \
     libssl1.0-dev libbz2-dev libosmesa6-dev libxmu6 libxmu-dev \
-    libglu1-mesa-dev libgles2-mesa-dev libegl1-mesa-dev libdbus-1-dev \
-    libpulse-dev
+    libglu1-mesa-dev libgles2-mesa-dev libegl1-mesa-dev \
+    pulseaudio dbus-x11 libavcodec-dev libavformat-dev \
+    libavutil-dev libswresample-dev  libswscale-dev libdbus-1-dev \
+    libpulse-dev clang
 ```
 
 If you using a version prior to **Ubuntu 17.04** or **Debian Sid**, replace `libssl1.0-dev` with `libssl-dev`.