Add ninja-build to prerequisites

Recent versions of QEMU needs ninja-build when building, so add that
to prerequisites.

Reviewed-by: Jerome Forissier <jerome@forissier.org>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>

building/prerequisites.rst

@@ -24,7 +24,7 @@ Install the following packages regardless of what target you will use in the end
             expect flex ftp-upload gdisk iasl libattr1-dev libcap-dev \
             libfdt-dev libftdi-dev libglib2.0-dev libgmp-dev libhidapi-dev \
             libmpc-dev libncurses5-dev libpixman-1-dev libssl-dev libtool make \
-            mtools netcat python-crypto python3-crypto python-pyelftools \
+            mtools netcat ninja-build python-crypto python3-crypto python-pyelftools \
             python3-pycryptodome python3-pyelftools python-serial python3-serial \
             rsync unzip uuid-dev xdg-utils xterm xz-utils zlib1g-dev