configure: Add missing <string.h> to avoid bogus warning

configure: Add missing <string.h> to avoid bogus warning

This warning has nothing to do with what the code is trying to test.
This isn't testing if memset(3) is available, so just silence the
warning by including an appropriate header.

--
 # uname
 Linux
 # grep memset config.log -A2 | head -3
 /tmp/fio-conf-23441-58170-6422.c:5:3: warning: implicit declaration of function 'memset' [-Wimplicit-function-declaration]
    memset(&cid, 0, sizeof(cid));
    ^~~~~~

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