Fix LibevQuicAsyncUDPSocket build on macOS (recvmmsg)

Summary:
`recvmmsg` is a Linux-only syscall and is absent from Darwin's libc. `LibevQuicAsyncUDPSocket::recvmmsg` guarded the direct `::recvmmsg` call with only `#if !FOLLY_MOBILE`, but macOS desktop is not `FOLLY_MOBILE`, so the branch was compiled on Mac builds. With no global `::recvmmsg`, the identifier resolved to the class's own member function, producing `no member named 'recvmmsg' in the global namespace` and blocking any macOS app that links MNS/mvfst's libev QUIC (HTTP/3) path (e.g. the Remote Desktop Mac companion).

Exclude `__APPLE__` from the guard so macOS falls through to the existing `recvmsg` loop fallback, matching how folly's own `netops::recvmmsg` degrades on platforms lacking the syscall.

Reviewed By: kvtsoy

Differential Revision: D112124811

fbshipit-source-id: ba7805cd42a49d992b7589e0f05b6947d8157119

Author: crytaljin

quic/common/udpsocket/LibevQuicAsyncUDPSocket.cpp

@@ -682,7 +682,9 @@ int LibevQuicAsyncUDPSocket::recvmmsg(
     unsigned int vlen,
     unsigned int flags,
     struct timespec* timeout) {
-#if !FOLLY_MOBILE
+// recvmmsg is a Linux-only syscall; it is absent from Darwin's libc, so on
+// macOS (which is not FOLLY_MOBILE) fall through to the recvmsg loop below.
+#if !FOLLY_MOBILE && !defined(__APPLE__)
   if (reinterpret_cast<void*>(::recvmmsg) != nullptr) {
     return ::recvmmsg(fd_, msgvec, vlen, (int)flags, timeout);
   }