Don't ever support named struct, enum, variant declaration within structures

Don't ever support named struct, enum, variant declaration within structures

Even gcc emits a warning:

blah.c:
struct blah {
	struct bloh { int a; };
	struct bloh a;
};

gcc -c -o blah blah.c
blah.c:4: warning: declaration does not declare anything

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>