make ifaddrs.h expose sys/socket.h

the getifaddrs interface seems to have been invented by glibc, and
they expose socket.h, so for us not to do so is just gratuitous
incompatibility with the interface we're mimicing.

include/ifaddrs.h

@@ -7,6 +7,7 @@ extern "C" {
 
 #include <features.h>
 #include <netinet/in.h>
+#include <sys/socket.h>
 
 struct ifaddrs {
 	struct ifaddrs *ifa_next;