doc: reference a magnum-extras MSYS package as well.

doc/building.dox

@@ -164,8 +164,9 @@ which will build both a 32bit and a 64bit version. See the
 [MSYS2 Wiki](https://github.com/msys2/msys2/wiki/Creating-Packages) for more
 information.
 
-There are also MSYS2 packages for @ref building-plugins-packages-msys "Magnum Plugins"
-and @ref building-integration-packages-msys "Magnum Integration".
+There are also MSYS2 packages for @ref building-plugins-packages-msys "Magnum Plugins",
+@ref building-extras-packages-msys "Magnum Extras" and
+@ref building-integration-packages-msys "Magnum Integration".
 
 @subsection building-packages-deb Packages for Debian, Ubuntu and derivatives