[LyX/master] Fix compilation error with clang's libc++

[Jean-Marc Lasgouttes]

commit 626f8d4448608764a76f6eb209a8fef9507ab8b4
Author: Jean-Marc Lasgouttes <lasgout...@lyx.org>
Date:   Fri Jan 8 09:51:32 2016 +0100

    Fix compilation error with clang's libc++
    
    The bug is probably fixed for clang 3.7.1 or 3.8.0

diff --git a/src/TocBackend.h b/src/TocBackend.h
index d5dfae4..cc5e429 100644
--- a/src/TocBackend.h
+++ b/src/TocBackend.h
@@ -127,6 +127,9 @@ private:
 class Toc : public std::vector<TocItem>
 {
 public:
+       // This is needed to work around a libc++ bug
+       // https://llvm.org/bugs/show_bug.cgi?id=24137
+       Toc() {}
        typedef std::vector<TocItem>::const_iterator const_iterator;
        typedef std::vector<TocItem>::iterator iterator;
        const_iterator item(DocIterator const & dit) const;