[libsigcplusplus] visit_each: Prototype trackable as struct, not class.

[Murray Cumming <murrayc src gnome org>]

commit be91e8a4f8b785f1a68350818f45644a1c2fe397
Author: Murray Cumming <murrayc murrayc com>
Date:   Fri Apr 15 20:17:44 2016 +0200

    visit_each: Prototype trackable as struct, not class.
    
    clang++ rightly complains that it cannot be both a struct and a class.

 sigc++/visit_each.h |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/sigc++/visit_each.h b/sigc++/visit_each.h
index 6c5de14..5ef17e8 100644
--- a/sigc++/visit_each.h
+++ b/sigc++/visit_each.h
@@ -24,7 +24,7 @@
 namespace sigc
 {
 
-class trackable;
+struct trackable;
 
 #ifndef DOXYGEN_SHOULD_SKIP_THIS
 namespace internal