configure: Add void* cast to avoid bogus warning

configure: Add void* cast to avoid bogus warning

This warning has nothing to do with what the code is trying to test.
This test code should keep it volatile, so cast to void* to silence
a warning on discarding volatile.

--
 # uname
 Linux
 # grep memset config.log -A2 | head -3
 /tmp/fio-conf-15226-60011-1419.c:6:10: warning: passing argument 1 of 'memset' discards 'volatile' qualifier from pointer target type [-Wdiscarded-qualifiers]
    memset(&cid, 0, sizeof(cid));
           ^

Signed-off-by: Tomohiro Kusumi <tkusumi@tuxera.com>
Signed-off-by: Jens Axboe <axboe@fb.com>