Avoid redefinition of mmsghdr with wasm32 in folly/NetOps.h

Summary:
wasi-libc already defines mmsghdr struct https://github.com/CraneStation/wasi-libc/blob/d4db3fa2120b27e01aac8f5f1b749dbbe1344880/libc-top-half/musl/include/sys/socket.h#L33

In folly we currently check for MSG_WAITFORONE to avoid redefinition, but MSG_WAITFORONE and bunch of other things are excluded from wasi-libc. So no to get to deep into rabbit hole just extending exclusion for wasm32.

Reviewed By: yfeldblum

Differential Revision: D19269397

fbshipit-source-id: 1bdd8344a2bec8a0b5be48bfedc94cc0d9a8635a

folly/net/NetOps.h

@@ -72,7 +72,7 @@
 #define UDP_MAX_SEGMENTS (1 << 6UL)
 #endif
 
-#ifndef MSG_WAITFORONE
+#if !defined(MSG_WAITFORONE) && !defined(__wasm32__)
 struct mmsghdr {
   struct msghdr msg_hdr;
   unsigned int msg_len;