[gtkmm-documentation] Add parentheses to silence clang-tidy.

[Murray Cumming <murrayc src gnome org>]

commit 65cdd7f5b72814979d4f8c59ab6b3b4db0d477c4
Author: Murray Cumming <murrayc murrayc com>
Date:   Mon Feb 8 18:49:39 2016 +0100

    Add parentheses to silence clang-tidy.

 examples/book/giomm/usage/usage.cc |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/examples/book/giomm/usage/usage.cc b/examples/book/giomm/usage/usage.cc
index 9539d23..c436065 100644
--- a/examples/book/giomm/usage/usage.cc
+++ b/examples/book/giomm/usage/usage.cc
@@ -34,7 +34,7 @@ static goffset get_size_recursively(const Glib::RefPtr<Gio::File>& file)
       auto children = file->enumerate_children();
       Glib::RefPtr<Gio::FileInfo> child_info;
 
-      while(child_info = children->next_file())
+      while((child_info = children->next_file()))
       {
         goffset size = get_size_recursively(
             file->get_child(child_info->get_name()));