Add fileno_unlocked to support thread sanitizer.

Bug: 25392375
Change-Id: If3f92a0e08a53f4b59e01397e8efa307d8572349
diff --git a/libc/stdio/stdio_ext.cpp b/libc/stdio/stdio_ext.cpp
index fea44f6..310076a 100644
--- a/libc/stdio/stdio_ext.cpp
+++ b/libc/stdio/stdio_ext.cpp
@@ -99,3 +99,7 @@
 int ferror_unlocked(FILE* fp) {
   return __sferror(fp);
 }
+
+int fileno_unlocked(FILE* fp) {
+  return __sfileno(fp);
+}