add missing includes to fix mac build (maybe)

Change-Id: Id2712940c4929f3a8b3ba5d4e9e03bb8034747ee

edify/expr.h

@@ -17,6 +17,8 @@
 #ifndef _EXPRESSION_H
 #define _EXPRESSION_H
 
+#include <unistd.h>
+
 #include "yydefs.h"
 
 #define MAX_STRING_LEN 1024

edify/lexer.l

@@ -15,6 +15,8 @@
  * limitations under the License.
  */
 
+#include <string.h>
+
 #include "expr.h"
 #include "yydefs.h"
 #include "parser.h"