compilation fix for gcc 4.7.4 under linux cent OS 6.7

src/main_utils.cpp

@@ -5,6 +5,7 @@
  *      Author: vlegrand
  */
 
+#include <unistd.h>
 #include <sys/types.h>
 #include <sys/sysctl.h>
 #include <sys/stat.h>