libceph: remove unneeded stddef.h include

This was causing a build failure for openrisc when using musl and
gcc 5.4.0 since the file is not available in the toolchain.

It doesnt seem this is needed and removing it does not cause any build
warnings for me.

Signed-off-by: Stafford Horne <shorne@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>