Make __set_errno hidden in asm.

This fixes the build after the -Bsymbolic change.

Bug: 16853291
Change-Id: I989c9fec3c32e0289ea257a3bd2b7fd2709b6ce2
diff --git a/libc/arch-x86_64/syscalls/mmap.S b/libc/arch-x86_64/syscalls/mmap.S
index 7b9f6e0..44189a9 100644
--- a/libc/arch-x86_64/syscalls/mmap.S
+++ b/libc/arch-x86_64/syscalls/mmap.S
@@ -2,6 +2,8 @@
 
 #include <private/bionic_asm.h>
 
+    .hidden __set_errno
+
 ENTRY(mmap)
     movq    %rcx, %r10
     movl    $__NR_mmap, %eax