Fix a parser bug in struct initialization.

[gergo-]

When initializing a struct containing a struct via a variable (see
test/initializer.c, function test_struct()):

struct foo { int a; int b; } f = { 1, 2 };
struct { struct foo f; } b = { f };

8cc failed with a type error:
[ERROR] parse.c:617: (null): incompatible kind: <(struct (int) (int))>

The read_initializer_elem function did not expect an identifier as a valid
struct member initializer. This patch seems to fix that (make fulltest
passes with the above-mentioned test added).