[libsigcplusplus] tests: Include cstring instead of string.h

[Murray Cumming <murrayc src gnome org>]

commit ac2657791b7a94162cb9addacc5c0cb15913dcf6
Author: Murray Cumming <murrayc murrayc com>
Date:   Fri Apr 21 14:50:28 2017 +0200

    tests: Include cstring instead of string.h
    
    Noticed by clang-tidy.

 tests/test_copy_invalid_slot.cc |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/tests/test_copy_invalid_slot.cc b/tests/test_copy_invalid_slot.cc
index 48bbfd8..6b85f60 100644
--- a/tests/test_copy_invalid_slot.cc
+++ b/tests/test_copy_invalid_slot.cc
@@ -4,7 +4,7 @@
 
 #include "testutilities.h"
 #include <sigc++/sigc++.h>
-#include <string.h>
+#include <cstring>
 
 namespace
 {