Actually define SS_NOPRIV.

Because in cd52bc19d9de6bc we made it just like all the others
except for the fact that the others already existed and this
one didn't.
Give caretx.c an explicit rule in vms/descrip_mms.template.

Without an explicit rule it gets build with CFLAGS rather than
CORECFLAGS, which could make it miss important things, such as
PERL_CORE being defined.
Only prototype should_warn_nl under PERL_CORE.

Otherwise the prototype is seen in places where the function
itself is not defined, which is illegal for inline functions.

Follow-up to 7cb3f9598b37fc.