Fix: Generate storage class for variable declaration

[ArthurPV]

The following code ends with a segfault because the variable stderr do not use the extern storage class.

Minimal version:

[[noreturn]] extern void __assert_fail(const char* __assertion, const char* __file, unsigned int __line, const char* __function);
[[noreturn]] extern void __assert(const char* __assertion, const char* __file, int __line);
typedef long int __off_t;
typedef long int __off64_t;
typedef __off64_t __loff_t;
typedef int __sig_atomic_t;
struct _IO_FILE;
typedef struct _IO_FILE __FILE;
struct _IO_marker;
struct _IO_codecvt;
struct _IO_wide_data;
typedef void _IO_lock_t;
typedef struct _IO_FILE FILE;
int main();

struct _IO_FILE {
    int _flags;
    char* _IO_read_ptr;
    char* _IO_read_end;
    char* _IO_read_base;
    char* _IO_write_base;
    char* _IO_write_ptr;
    char* _IO_write_end;
    char* _IO_buf_base;
    char* _IO_buf_end;
    char* _IO_save_base;
    char* _IO_backup_base;
    char* _IO_save_end;
    struct _IO_marker* _markers;
    struct _IO_FILE* _chain;
    int _fileno;
    int _flags2;
    __off_t _old_offset;
    unsigned short int _cur_column;
    signed char _vtable_offset;
    char _shortbuf[1];
    _IO_lock_t* _lock;
    __off64_t _offset;
    struct _IO_codecvt* _codecvt;
    struct _IO_wide_data* _wide_data;
    struct _IO_FILE* _freeres_list;
    void* _freeres_buf;
    struct _IO_FILE** _prevchain;
    int _mode;
    char _unused2[80];
};
FILE* stderr;
int main() {
    (0 && "error") ? ((void)0) : __assert_fail("0 && \"error\"", "/a/b/c", 49, __func__);
}