Fix IPv6 check on FreeBSD by including netinet/in.h

I noticed IPv6 support was being disabled on FreeBSD; it seems the
configure script needs netinet/in.h so in6_addr is known.

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/configure b/configure
index 78a6ccb..89405cd 100755
--- a/configure
+++ b/configure
@@ -1098,6 +1098,7 @@
 cat > $TMPC << EOF
 #include <sys/types.h>
 #include <sys/socket.h>
+#include <netinet/in.h>
 #include <netdb.h>
 #include <stdio.h>
 int main(int argc, char **argv)