Add ninja-build to the packages list to install

The package ninja-build should be installed automatically as a meson
dependency, but some users need to install a newer meson from pip3, so
ninja must be installed explicitly.

BUILD.md

@@ -43,7 +43,7 @@ Install the required packages from your package manager.
 sudo apt install ffmpeg libsdl2-2.0.0
 
 # client build dependencies
-sudo apt install make gcc pkg-config meson \
+sudo apt install make gcc pkg-config meson ninja-build \
                  libavcodec-dev libavformat-dev libavutil-dev \
                  libsdl2-dev