lcfs-verity: Add missing include

This doesn't matter much since we pull in stddef.h in the
normal path from elsewhere, but it allows me to run Rust's
`bindgen` tool directly against this header.

Signed-off-by: Colin Walters <[email protected]>

libcomposefs/lcfs-fsverity.h

@@ -15,6 +15,7 @@
    along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
 
 #include <stdint.h>
+#include <stddef.h>
 
 typedef struct FsVerityContext FsVerityContext;