Add ninja-build to Debian build dependencies

I needed to manually install it --hence adding it to the list of dependencies is a good idea.

README.md

@@ -309,7 +309,7 @@ sudo pacman -S qt5-base \
 ```bash
 sudo apt install --no-install-recommends g++ cmake make zlib1g-dev libssl-dev libolm-dev liblmdb-dev libcmark-dev nlohmann-json3-dev libspdlog-dev libevent-dev libcurl4-openssl-dev libre2-dev libxcb-ewmh-dev asciidoc-base \
 qt{base,declarative,tools,multimedia,quickcontrols2-}5-dev libqt5svg5-dev qt5keychain-dev qml-module-qt{gstreamer,multimedia,quick-extras,-labs-settings,-labs-platform,graphicaleffects,quick-controls2,quick-particles2} \
-libgstreamer1.0-dev libgstreamer-plugins-{base,bad}1.0-dev qtgstreamer-plugins-qt5 libnice-dev
+libgstreamer1.0-dev libgstreamer-plugins-{base,bad}1.0-dev qtgstreamer-plugins-qt5 libnice-dev ninja-build
 ```
 lmdb++-dev is too old so bundled lmdbxx must be used.  
 libspdlog-dev in debian bullseye is too old (without backporting) so requires using hunter to use bundled spdlog.