params: forward declare struct ngl_node
params: forward declare struct ngl_node

struct node_class (from nodes.h) depends on struct node_param (from
params.h), and at the same time node_param needs ngl_node (from
nodes.h). This forward declaration addresses the cyclical dependency and
allows including params.h without nodes.h.

gopro/gopro-lib-node.gl,gopro/gopro-lib-node.gl,gopro/gopro-lib-node.gl,gopro/gopro-lib-node.gl