Correctly include netinet/in.h (#2315)

Motivation:

IPV6_RECVPKTINFO and IPV6_PKTINFO are not currently directly included.
On Apple platforms, these are found by including netinet/in.h, so we
should include that header directly.

Modifications:

- Add the correct include to shims.c

Result:

Better macOS builds

Sources/CNIODarwin/shim.c

@@ -20,6 +20,7 @@
 #include <errno.h>
 #include <assert.h>
 #include <netinet/ip.h>
+#include <netinet/in.h>
 
 int CNIODarwin_sendmmsg(int sockfd, CNIODarwin_mmsghdr *msgvec, unsigned int vlen, int flags) {
     // Some quick error checking. If vlen can't fit into int, we bail.