include: Had to include <cstring> in a few places to be able to compile cppcheck

src/checkclass.cpp

@@ -24,6 +24,7 @@
 
 #include <locale>
 
+#include <cstring>
 #include <string>
 #include <sstream>
 #include <algorithm>

src/checkmemoryleak.cpp

@@ -22,6 +22,7 @@
 #include "errormessage.h"
 
 #include <algorithm>
+#include <cstring>
 #include <iostream>
 #include <sstream>
 

src/filelister.cpp

@@ -20,6 +20,7 @@
 #include "filelister.h"
 #include <sstream>
 #include <vector>
+#include <cstring>
 #include <string>
 #include <cctype>
 #include <algorithm>