lsblk: fix unknown type `stat` caused by missing header

lsblk: fix unknown type `stat` caused by missing header

The structure `blkdev_cxt` has a `struct stat` member embedded, whose
size may not be known on some systems because of a missing include for
"sys/stat.h". On glibc-based systems, this header is included
transitively via "sys/statvfs.h", but on musl-based systems it is not.

Fix the resulting compile error due to unknown size of the struct by
including "sys/stat.h".

Signed-off-by: Patrick Steinhardt <ps@pks.im>