cxgb4: move DCB version extern to header file

Move the DCB version string array extern to header file.

Fixes following sparse warning:
cxgb4_dcb.c:13:12: warning: symbol 'dcb_ver_array' was not declared.
Should it be static?

Fixes: ebddd97a ("cxgb4: add support to display DCB info")
Signed-off-by: Rahul Lakkireddy <rahul.lakkireddy@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>