servo: Merge #4669 - Add libosmesa6-dev to debian dependencies (from servo:glennw-patch-1); r=larsbergstrom

Needs to be updated for arch / fedora too, but I'm not sure what the relevant packages are.

Source-Repo: https://github.com/servo/servo
Source-Revision: 0a787ddb671afda148734c6e499bce29fc33a743

servo/README.md

@@ -28,7 +28,7 @@ On Debian-based Linuxes:
 sudo apt-get install curl freeglut3-dev \
     libfreetype6-dev libgl1-mesa-dri libglib2.0-dev xorg-dev \
     msttcorefonts gperf g++ cmake python-virtualenv \
-    libssl-dev libbz2-dev
+    libssl-dev libbz2-dev libosmesa6-dev
 ```
 
 On Fedora: