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>