Downstream-only patch to restore the extern ABI for functions
like fprintf that use the FILE * type.  Rebuilds of applications
receive ABI change reports because of this installed header change
(indirect subtype change in libabigail terms), and given that
this part of struct _IO_FILE is strictly internal, there is no
need to expose this change to installed headers.

diff --git a/libio/bits/types/struct_FILE.h b/libio/bits/types/struct_FILE.h
index 0e73f89f813ef3b8..f7f756a701ce0e93 100644
--- a/libio/bits/types/struct_FILE.h
+++ b/libio/bits/types/struct_FILE.h
@@ -71,9 +71,14 @@ struct _IO_FILE
   struct _IO_FILE *_chain;
 
   int _fileno;
+#ifdef _LIBC
   int _flags2:24;
   /* Fallback buffer to use when malloc fails to allocate one.  */
   char _short_backupbuf[1];
+#else
+  /* Legacy ABI for ABI checking outside of glibc.  */
+  int _flags2;
+#endif
   __off_t _old_offset; /* This used to be _offset but it's too small.  */
 
   /* 1+column number of pbase(); 0 is unknown. */