Unhide __set_errno for backwards compatibility.

This was in <errno.h>, and people called it :-(

Change-Id: I038490be77eb9372b3f31305ec580fa7b09c983e
diff --git a/libc/bionic/__set_errno.cpp b/libc/bionic/__set_errno.cpp
index 4e72093..af6a68e 100644
--- a/libc/bionic/__set_errno.cpp
+++ b/libc/bionic/__set_errno.cpp
@@ -31,7 +31,8 @@
 // This function is called from our assembler syscall stubs.
 // C/C++ code should just assign 'errno' instead.
 
-extern "C" __LIBC_HIDDEN__ int __set_errno(int n) {
+// TODO: this should be __LIBC_HIDDEN__ but was exposed in <errno.h> in the NDK.
+extern "C" int __set_errno(int n) {
   errno = n;
   return -1;
 }