Add support of extern global variable

KLEE error on files with extern global
For example on project [FRRouting/frr](https://github.com/FRRouting/frr)

```bash
KLEE: ERROR: Unable to load symbol(MTYPE_TMP) while initializing globals
```

Minimal reproduce example

```cpp
extern int b;

int fun() {
    return b;
}
```