Fix x86_64 __set_tls.c build failure.

Change-Id: I697b8ab4ce781d7aebeddb1391b63c653511bbcb
diff --git a/libc/arch-x86_64/bionic/__set_tls.c b/libc/arch-x86_64/bionic/__set_tls.c
index 6845444..10fd36f 100644
--- a/libc/arch-x86_64/bionic/__set_tls.c
+++ b/libc/arch-x86_64/bionic/__set_tls.c
@@ -26,6 +26,7 @@
  * SUCH DAMAGE.
  */
 
+#include <sys/cdefs.h>
 #include <asm/prctl.h>
 #include <stdint.h>