Revert "diff: eliminate a useless lseek"

This changes breaks when one of the files is stdin

This reverts commit fa977a3b2b.

Reported by:	olivier

usr.bin/diff/diffreg.c

@@ -490,6 +490,7 @@ opentemp(const char *f)
 		}
 	}
 	close(ifd);
+	lseek(ofd, (off_t)0, SEEK_SET);
 	return (fdopen(ofd, "r"));
 }