error: 'stddef.h' file not found on 12

I am getting net-im/ricochet failures on 12:

/usr/include/c++/v1/cstddef:44:15: fatal error: 'stddef.h' file not found
#include_next <stddef.h>
               ^~~~~~~~~~

cstddef is including stddef.h that has to be present.

11 has both of them:

# ls /usr/include/c++/v1/*stddef*
/usr/include/c++/v1/cstddef  /usr/include/c++/v1/stddef.h

why isn't it present in 12?

Yuri