netlink: extack: avoid parenthesized string constant warning

NL_SET_ERR_MSG() and NL_SET_ERR_MSG_ATTR() lead to the following warning
in newer versions of gcc:
  warning: array initialized from parenthesized string constant

Just remove the parentheses, they're not needed in this context since
anyway since there can be no operator precendence issues or similar.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>