Non-extern global variable without initializer is not marked as the definition

`a` and `b` should be equivalent:
```c
extern int a;
int a = 0;

extern int b;
int b;
```

```
variable: 'int'
 extern name: a
 definition: 0x7

variable: 'int'
 name: a
 init:
  int_literal: 'int' (value: 0)

variable: 'int'
 extern name: b // missing definition pointing to the decl below

variable: 'int'
 name: b
```