Fix compilation on platforms without SO_RCVBUFFORCE.

Author: sunfishcode

src/backend/libc/net/sockopt.rs

@@ -344,6 +344,7 @@ pub(crate) fn set_socket_recv_buffer_size(fd: BorrowedFd<'_>, size: usize) -> io
     setsockopt(fd, c::SOL_SOCKET, c::SO_RCVBUF, size)
 }
 
+#[cfg(any(linux_kernel, target_os = "fuchsia", target_os = "redox"))]
 #[inline]
 pub(crate) fn set_socket_recv_buffer_size_force(fd: BorrowedFd<'_>, size: usize) -> io::Result<()> {
     let size: c::c_int = size.try_into().map_err(|_| io::Errno::INVAL)?;

src/net/sockopt.rs

@@ -401,6 +401,7 @@ pub fn set_socket_recv_buffer_size<Fd: AsFd>(fd: Fd, value: usize) -> io::Result
 /// See the [module-level documentation] for more.
 ///
 /// [module-level documentation]: self#references-for-get_socket_-and-set_socket_-functions
+#[cfg(any(linux_kernel, target_os = "fuchsia", target_os = "redox"))]
 #[inline]
 #[doc(alias = "SO_RCVBUFFORCE")]
 pub fn set_socket_recv_buffer_size_force<Fd: AsFd>(fd: Fd, value: usize) -> io::Result<()> {