build: fix some compiler warnings

Don't know whether you would fix all of them like this.

Some additional notes:
* sys-utils/losetup.c: "set but never used"
... maybe a good motivation to give some love to "losetup --verbose"

* sys-utils/switch_root.c:  "comparison between signed and unsigned"
... could be generally fixed in include/statfs_magic.h for a better future

* there is a one more serious warning on big endian systems but I don't see how this could happen:
<pre>
libblkid/src/superblocks/zfs.c: In function 'probe_zfs':
libblkid/src/superblocks/zfs.c:214:27: warning: 'ub_offset' may be used uninitialized in this function [-Wmaybe-uninitialized]
  if (blkid_probe_set_magic(pr, ub_offset,
                           ^
</pre>