Include <stddef.h>, for size_t when libiberty.h doesn't define size_t.

lib/bbitset.h

@@ -23,6 +23,7 @@ Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.  */
 
 #include <stdbool.h>
 #include <limits.h>
+#include <stddef.h>
 
 /* Currently we support five flavours of bitsets:
    BITSET_ARRAY:  Array of bits (fixed size, fast for dense bitsets).