support/graph-depends: fix package names starting with a non-alpha

Graphviz' dot utility does not like nodes which names does not start
with an ^[[:alpha:]], i.e. 18xx-ti-utils would cause grievance:

    Warning: syntax ambiguity - badly delimited number '18x' in line 4 [...]/graph-depends.dot splits into two tokens
    Warning: syntax ambiguity - badly delimited number '18x' in line 5 [...]/graph-depends.dot splits into two tokens
    Warning: syntax ambiguity - badly delimited number '18x' in line 6 [...]/graph-depends.dot splits into two tokens
    Warning: syntax ambiguity - badly delimited number '18x' in line 7 [...]/graph-depends.dot splits into two tokens

Prefix nodes with an underscore to fix that.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>