Reordered struct members to not waste memory due to padding.

Before:
``` C
typedef struct {
    uint8_t src_l2addr_len;
    uint8_t dst_l2addr_len;
    kernel_pid_t if_pid;    // <-- 16 bit, is aligned to 16 bit
    uint8_t flags;
    uint8_t __padding_byte; // <-- Inserted to fulfill padding requirements
    int16_t rssi;           // <-- 16 bit, is NOT aligned to 16 bit
    uint8_t lqi;
    uint8_t __padding_byte2;// <-- Inserted to fulfill padding requirements
} gnrc_netif_hdr_t;
```

Now:
``` C
typedef struct {
    uint8_t src_l2addr_len;
    uint8_t dst_l2addr_len;
    kernel_pid_t if_pid;    // <-- 16 bit, is aligned to 16 bit
    uint8_t flags;
    uint8_t lqi;
    int16_t rssi;           // <-- 16 bit, is aligned to 16 bit
} gnrc_netif_hdr_t;
```

When build for the `bluepill` board, the new layout reduces the size by 2 bytes.