Include <sys/stat.h> in src/util.c

On FreeBSD 11-CURRENT this fixes the compilation failure:

src/util.c: In function 'file_write':
src/util.c:256: error: 'S_IRUSR' undeclared (first use in this function)
src/util.c:256: error: (Each undeclared identifier is reported only once
src/util.c:256: error: for each function it appears in.)
src/util.c:256: error: 'S_IWUSR' undeclared (first use in this function)
*** Error code 1
diff --git a/src/util.c b/src/util.c
index 51f2492..3cf9ea9 100644
--- a/src/util.c
+++ b/src/util.c
@@ -20,6 +20,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <sys/ioctl.h>
+#include <sys/stat.h>
 #include <sys/time.h>
 #include <sys/types.h>
 #include <sys/wait.h>