e1000: fix build on Ubuntu with _FORTIFY_SOURCE

There was a pointer cast warning on Ubuntu since _FORTIFY_SOURCE has been reenabled.

_FORTIFY_SOURCE had been disabled by 4a244704
and reenabled by 84958305.
Signed-off-by: NThomas Monjalon <thomas@monjalon.net>
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>

hw/e1000.c

@@ -642,7 +642,7 @@ e1000_receive(VLANClientState *nc, const uint8_t *buf, size_t size)
 
     if (vlan_enabled(s) && is_vlan_packet(s, buf)) {
         vlan_special = cpu_to_le16(be16_to_cpup((uint16_t *)(buf + 14)));
-        memmove((void *)(buf + 4), buf, 12);
+        memmove((uint8_t *)buf + 4, buf, 12);
         vlan_status = E1000_RXD_STAT_VP;
         vlan_offset = 4;
         size -= 4;