manual: add requirements for graph generation

Add "graphviz" and "python-matplotlib" as requirements for graph
generation.

Signed-off-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Reported-by: Dallas Clement <dallas.a.clement@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Yann E. MORIN <yann.morin.1998@free.fr>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

docs/manual/prerequisite.txt

@@ -77,3 +77,7 @@ development context (further details: refer to xref:download-infra[]).
 ** +w3m+
 ** +python+ with the +argparse+ module (automatically present in 2.7+ and 3.2+)
 ** +dblatex+ (required for the pdf manual only)
+
+* Graph generation tools:
+** +graphviz+ to use 'graph-depends' and '<pkg>-graph-depends'
+** +python-matplotlib+ to use 'graph-build'