Include config.h in gssdp-socket-functions.c

PKTINFO socket option was not set because of missing HAVE_PKTINFO
definition.

https://bugzilla.gnome.org/show_bug.cgi?id=733651
Use int consistently for packet info interface index

g_value_set_uint() with an int argument is not good: use int consistently
(based on the kernel value being an int).

https://bugzilla.gnome.org/show_bug.cgi?id=733651
Fix pkt-address property in PktInfoMessage

Signed-off-by: Jens Georg <mail@jensge.org>

https://bugzilla.gnome.org/show_bug.cgi?id=733651
Check arguments in PktInfoMessage public functions

Signed-off-by: Jens Georg <mail@jensge.org>