Hide __set_tls.

Bug: 11156955
Change-Id: If82c10ae412651361d09591017d1c3a411bd6f0b
diff --git a/libc/arch-arm64/bionic/__set_tls.c b/libc/arch-arm64/bionic/__set_tls.c
index 4eb3ade..0d88d11 100644
--- a/libc/arch-arm64/bionic/__set_tls.c
+++ b/libc/arch-arm64/bionic/__set_tls.c
@@ -26,6 +26,8 @@
  * SUCH DAMAGE.
  */
 
-void __set_tls(void* tls) {
+#include <sys/cdefs.h>
+
+__LIBC_HIDDEN__ void __set_tls(void* tls) {
   asm("msr tpidr_el0, %0" : : "r" (tls));
 }