Fix recv.cpp so it can build with clang.

Change-Id: Iac3e676aa256837333240219bcadbdc31985b5dd
diff --git a/libc/bionic/recv.cpp b/libc/bionic/recv.cpp
index 60f264d..061cd46 100644
--- a/libc/bionic/recv.cpp
+++ b/libc/bionic/recv.cpp
@@ -26,6 +26,7 @@
  * SUCH DAMAGE.
  */
 
+#undef _FORTIFY_SOURCE
 #include <sys/socket.h>
 
 ssize_t recv(int socket, void *buf, size_t len, int flags) {