doc: getting_started: Require ninja on macOS

Since we're switching to ninja as a default generator for CMake, require
the ninja package as part of the requirements on macOS.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>

doc/getting_started/installation_mac.rst

@@ -57,7 +57,7 @@ Install tools to build Zephyr binaries:
 
 .. code-block:: console
 
-   $ brew install cmake dfu-util doxygen qemu dtc python3 gperf
+   $ brew install cmake ninja dfu-util doxygen qemu dtc python3 gperf
    $ curl -O 'https://bootstrap.pypa.io/get-pip.py'
    $ ./get-pip.py
    $ rm get-pip.py