fix "headercheck":

part of https://github.com/assimp/assimp/issues/534

code/StringComparison.h

@@ -51,6 +51,9 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 #define INCLUDED_AI_STRING_WORKERS_H
 
 #include "../include/assimp/ai_assert.h"
+#include <string.h>
+#include <stdint.h>
+#include <string>
 
 namespace Assimp	{