libstdc++: Fix name of macro in #undef directive

The macro that is defined is _GLIBCXX_NOT_FN_CALL_OP but the macro that
was named in the #undef directive was _GLIBCXX_NOT_FN_CALL. This fixes
the #undef.

	* include/std/functional (_GLIBCXX_NOT_FN_CALL_OP): Un-define after
	use.

libstdc++-v3/ChangeLog

 2020-02-04  Jonathan Wakely  <jwakely@redhat.com>
 
+	* include/std/functional (_GLIBCXX_NOT_FN_CALL_OP): Un-define after
+	use.
+
 	PR libstdc++/93562
 	* include/bits/unique_ptr.h (__uniq_ptr_impl::swap): Define.
 	(unique_ptr::swap, unique_ptr<T[], D>::swap): Call it.

libstdc++-v3/include/std/functional

@@ -953,7 +953,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       _GLIBCXX_NOT_FN_CALL_OP( const & )
       _GLIBCXX_NOT_FN_CALL_OP( && )
       _GLIBCXX_NOT_FN_CALL_OP( const && )
-#undef _GLIBCXX_NOT_FN_CALL
+#undef _GLIBCXX_NOT_FN_CALL_OP
 
     private:
       _Fn _M_fn;