Include string.h and libintl.h, as needed.

Include string.h and libintl.h, as needed.

* df/df.c: As above.
* df/main.c: As above.
* df/output.c: As above.
* fuse/guestmount.c: As above.
* inspector/virt-inspector.c: As above.
* rescue/virt-rescue.c: As above.
(cherry picked from commit 6740028b333840eec5e8e46e8512f8155728b037)