scripts/graph-build-time: clarify backend selection

This instruction in the middle of 'import' lines looks very strange.

Also, it was not obvious to me what the 'Agg' backend is.

Both things are actually correct, but it took a while to find out why.
So clarify with a comment to save someone else's time.

[Peter: fix s/soe/some/ typo]
Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Cc: Sascha Arthur <sascha.arthur@gmail.com>
Cc: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>