add missing includes to fix mac build (maybe)

Change-Id: Id2712940c4929f3a8b3ba5d4e9e03bb8034747ee
diff --git a/edify/lexer.l b/edify/lexer.l
index 2c4489c..fb2933b 100644
--- a/edify/lexer.l
+++ b/edify/lexer.l
@@ -15,6 +15,8 @@
  * limitations under the License.
  */
 
+#include <string.h>
+
 #include "expr.h"
 #include "yydefs.h"
 #include "parser.h"