Fix igmp function declarations

[scribam]

To match with the ones declared above and the function definition:

picotcp/modules/pico_igmp.c

Line 584 in 26af516

static int pico_igmp_process_in(struct pico_stack *S, struct pico_protocol *self, struct pico_frame *f)

picotcp/modules/pico_igmp.c

Line 607 in 26af516

static int pico_igmp_process_out(struct pico_stack *S, struct pico_protocol *self, struct pico_frame *f)

picotcp/modules/pico_igmp.c

Line 625 in 26af516

int pico_igmp_state_change(struct pico_stack *S, struct pico_ip4 *mcast_link, struct pico_ip4 *mcast_group, uint8_t filter_mode, struct pico_tree *_MCASTFilter, uint8_t state)

picotcp/modules/pico_igmp.h

Line 47 in 26af516

int pico_igmp_state_change(struct pico_stack *S, struct pico_ip4 *mcast_link, struct pico_ip4 *mcast_group, uint8_t filter_mode, struct pico_tree *_MCASTFilter, uint8_t state);