scripts: footprint: fix node size computation

fix issue with (no paths) != sum(<no paths>)

In some cases it was observed that item were being
added to the node_no_paths and these only had a
path of ':' hence no node was ever added.

This resulted in the size of (no paths) to not be
equal to the sum of it's children.

I am not sure if this size should be a child named
of no paths or if  this should be part of the
node_hidden_syms. I assumed it should be part
of node_hidden_syms

Signed-off-by: Kyle Micallef Bonnici <kylebonnici@hotmail.com>