pcap_sg: Fix various compiler warnings

Fix the following compiler warnings that occur when building with "-W -Wall":

pcap_sg.c: In function ‘__pcap_sg_inter_iov_hdr_read’:
pcap_sg.c:46:89: warning: unused parameter ‘type’ [-Wunused-parameter]
pcap_sg.c:47:19: warning: unused parameter ‘packet’ [-Wunused-parameter]
pcap_sg.c:47:34: warning: unused parameter ‘len’ [-Wunused-parameter]
pcap_sg.c: In function ‘__pcap_sg_inter_iov_data_read’:
pcap_sg.c:77:78: warning: unused parameter ‘len’ [-Wunused-parameter]
pcap_sg.c: In function ‘pcap_sg_prepare_access’:
pcap_sg.c:163:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
pcap_sg.c: In function ‘pcap_sg_prepare_close’:
pcap_sg.c:184:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
pcap_sg.c:180:39: warning: unused parameter ‘fd’ [-Wunused-parameter]
pcap_sg.c:180:58: warning: unused parameter ‘mode’ [-Wunused-parameter]

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>