volser: fix set-but-unused variable warning

volser: fix set-but-unused variable warning

restorevol reads the magic number from the dump, then does nothing
with it. Rather than not reading it at all, just mark the variable that
it is read into as unused to supress the compiler warning.

Change-Id: I371e2556e3661836f6256fac255ae79005d9ffaa
Reviewed-on: http://gerrit.openafs.org/5002
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>