[libsigcplusplus] tests: Initialize a member variable.

[Murray Cumming <murrayc src gnome org>]

commit 3026ef7c4e70d8dcffe821f9064652446b44db41
Author: Murray Cumming <murrayc murrayc com>
Date:   Fri Apr 21 14:53:06 2017 +0200

    tests: Initialize a member variable.
    
    Noticed by clang-tidy.

 tests/test_trackable.cc |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/tests/test_trackable.cc b/tests/test_trackable.cc
index cb55edb..bd48fe6 100644
--- a/tests/test_trackable.cc
+++ b/tests/test_trackable.cc
@@ -13,7 +13,7 @@ std::ostringstream result_stream;
 class my_class : public sigc::trackable
 {
 public:
-  int i;
+  int i = 0;
 
   void foo() { result_stream << i; }
 };