bfd: restore Threading menu entry in bfd.texi

I mistakenly vandalized bfd.texi in the commit
0c45feb159a14ca4cb50cfbf45eacaf5a6cecf2b by removing an entry in the
manual menu.  This commit reverts that thunk.
diff --git a/bfd/doc/bfd.texi b/bfd/doc/bfd.texi
index e177100..60061d5 100644
--- a/bfd/doc/bfd.texi
+++ b/bfd/doc/bfd.texi
@@ -199,6 +199,7 @@
 * typedef bfd::
 * Error reporting::
 * Initialization::
+* Threading::
 * Miscellaneous::
 * Memory Usage::
 * Sections::