[libsigcplusplus] tests: Implement a function.

[Murray Cumming <murrayc src gnome org>]

commit d1a04ee9b87bdac0a1d295710f49dd39cb6120e1
Author: Murray Cumming <murrayc murrayc com>
Date:   Tue Apr 18 17:56:52 2017 +0200

    tests: Implement a function.
    
    We don't actually call the function, but it avoids an inspection
    warning from Jetbrains CLion, and doesn't seem unreasonable.

 tests/test_size.cc |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/tests/test_size.cc b/tests/test_size.cc
index d49af99..cf94a8b 100644
--- a/tests/test_size.cc
+++ b/tests/test_size.cc
@@ -15,7 +15,7 @@ namespace
 {
 struct A
 {
-  void foo();
+  void foo() {}
 };
 } // end anonymous namespace