Add stat.h to editor

Fix on openbsd:
`src/tools/editor.c:55:36: error: 'S_IRWXU' undeclared (first use in
this function)`

src/tools/editor.c

@@ -35,6 +35,7 @@
  */
 
 #include <fcntl.h>
+#include <sys/stat.h>
 #include <sys/wait.h>
 #include <errno.h>
 #include <glib.h>