Avoid redefinition of mmsghdr with Android NDK unified headers

Summary:
Avoid redefinition of mmsghdr with newer Android NDK unified headers.

Unified headers unconditionally define `struct mmsghdr`, but the `sendmmsg` and `recvmmsg` system calls are conditional on API level. API 21 is the first level that supports them.

Reviewed By: yfeldblum

Differential Revision: D15344307

fbshipit-source-id: 6f03197cc346511ce7a02f1468c35313140a8fd7

folly/net/NetOps.h

@@ -72,7 +72,7 @@
 #define UDP_MAX_SEGMENTS (1 << 6UL)
 #endif
 
-#if !(FOLLY_HAVE_RECVMMSG || FOLLY_HAVE_SENDMMSG)
+#ifndef MSG_WAITFORONE
 struct mmsghdr {
   struct msghdr msg_hdr;
   unsigned int msg_len;