gnrc/netif/hdr.h: Optimized structure layout

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.

sys/include/net/gnrc/netif/hdr.h

@@ -85,8 +85,8 @@ typedef struct {
     uint8_t dst_l2addr_len;     /**< length of l2 destination address in byte */
     kernel_pid_t if_pid;        /**< PID of network interface */
     uint8_t flags;              /**< flags as defined above */
-    int16_t rssi;               /**< rssi of received packet in dBm (optional) */
     uint8_t lqi;                /**< lqi of received packet (optional) */
+    int16_t rssi;               /**< rssi of received packet in dBm (optional) */
 } gnrc_netif_hdr_t;
 
 /**