I think that void net_deinit(void); in network.h should be within the #ifdef HW_RVL / #endif block just over it.
The code is only defined in network_wii.c and compiling my code for GameCube will generate this
warning: undefined reference to `net_deinit'
Maybe (not sure), the other solution is to add this to lwip\network.c:
void net_deinit(void) {
	return;
}