c: Restore warning for incomplete structures declared in parameter list [PR117866]

c: Restore warning for incomplete structures declared in parameter list [PR117866]

In C23 mode the warning about declaring structures and union in
parameter lists was removed, because it is possible to redeclare
a compatible type elsewhere.  This is not the case for incomplete types,
so restore the warning for those types.

	PR c/117866

gcc/c/ChangeLog:
	* c-decl.cc (get_parm_info): Change condition for warning.

gcc/testsuite/ChangeLog:
	* gcc.dg/pr117866.c: New test.
	* gcc.dg/strub-pr118007.c: Adapt.