Fix handling of ident-less declarations

Fix handling of ident-less declarations

The rule for ident-less declaration is
	declaration -> declaration-specifiers ;
not
	declaration -> declaration-specifiers abstract-declarator;

IOW, struct foo; is OK and so's struct foo {int x; int y;} (and even
simply int; is allowed by syntax - it's rejected by constraints, but
that's a separate story), but not struct foo (void); and its ilk.

See C99 6.7p1 for syntax; C90 is the same in that area and gcc also
behaves the same way.  Unlike gcc I've made it a warning (gcc produces
a hard error for those).

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Christopher Li <sparse@chrisli.org>