StringUtils: add the right missing header.

code/StringUtils.h

@@ -41,7 +41,7 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 #define INCLUDED_AI_STRINGUTILS_H
 
 #include <sstream>
-#include <string.h>
+#include <stdarg.h>
 
 ///	@fn		ai_snprintf
 ///	@brief	The portable version of the function snprintf ( C99 standard ), which works on visual studio compilers 2013 and earlier.