Fix network_spgist.c build failures from missing AF_INET definition.

Fix network_spgist.c build failures from missing AF_INET definition.

AF_INET is apparently defined in something that's pulled in automatically
on Linux, but the buildfarm says that's not true everywhere.  Comparing
to network_gist.c suggests that including <sys/socket.h> ought to fix it,
and the POSIX standard concurs.