remove warning

keyboards/sixkeyboard/matrix.c

@@ -34,6 +34,7 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 #include "util.h"
 #include "matrix.h"
 #include "sixkeyboard.h"
+#include <string.h>
 
 /* matrix state(1:on, 0:off) */
 static matrix_row_t matrix[MATRIX_ROWS];