libc: tag missing functions in system headers.

This matches recent changes in the NDK header.
We enclose missing functions in #if 0 .. #endif blocks
with a clear "MISSING" in comments in order to locate
them later.

Change-Id: I87b3a62e777897e75c9243360fb0a82bcc53d9fb
diff --git a/libc/include/sys/fsuid.h b/libc/include/sys/fsuid.h
index 3257bc0..bc47e3d 100644
--- a/libc/include/sys/fsuid.h
+++ b/libc/include/sys/fsuid.h
@@ -33,8 +33,10 @@
 
 __BEGIN_DECLS
 
+#if 0 /* MISSING FROM BIONIC */
 extern int setfsuid(uid_t);
 extern int setfsgid(gid_t);
+#endif /* MISSING */
 
 __END_DECLS