Add missing header observed after linux-legacy was reverted.

Interestingly this has only been a problem on a clean RHEL6
install so I suspect the include was removed from one of the
standard system include headers.  We should be including it
explicitly anyway since it's used in both of these .c files.

lib/libzfs/libzfs_pool.c

@@ -37,6 +37,7 @@
 #include <strings.h>
 #include <unistd.h>
 #include <zone.h>
+#include <sys/stat.h>
 #include <sys/efi_partition.h>
 #include <sys/vtoc.h>
 #include <sys/zfs_ioctl.h>

lib/libzfs/libzfs_util.c

@@ -37,6 +37,7 @@
 #include <unistd.h>
 #include <ctype.h>
 #include <math.h>
+#include <sys/stat.h>
 #include <sys/mnttab.h>
 #include <sys/mntent.h>
 #include <sys/types.h>