doc: add graphviz imagemagick as build dependencies

'make htmldocs' requires graphviz and imagemagick (at least with Sphinx
3.5.4).
Signed-off-by: NHeinrich Schuchardt <xypron.glpk@gmx.de>

doc/build/gcc.rst

@@ -24,9 +24,9 @@ Depending on the build targets further packages maybe needed
 .. code-block:: bash
 
     sudo apt-get install bc bison build-essential coccinelle \
-      device-tree-compiler dfu-util efitools flex gdisk liblz4-tool \
-      libguestfs-tools libncurses-dev libpython3-dev libsdl2-dev libssl-dev \
-      lz4 lzma lzma-alone openssl python3 python3-coverage \
+      device-tree-compiler dfu-util efitools flex gdisk graphviz imagemagick \
+      liblz4-tool libguestfs-tools libncurses-dev libpython3-dev libsdl2-dev \
+      libssl-dev lz4 lzma lzma-alone openssl python3 python3-coverage \
       python3-pycryptodome python3-pyelftools python3-pytest \
       python3-sphinxcontrib.apidoc python3-sphinx-rtd-theme python3-virtualenv \
       swig