igb: Fix -Wunused-const-variable warning

If CONFIG_IGB_HWMON is n, gcc warns:

drivers/net/ethernet/intel/igb/e1000_82575.c:2765:17:
 warning: ‘e1000_emc_therm_limit’ defined but not used [-Wunused-const-variable=]
 static const u8 e1000_emc_therm_limit[4] = {
                 ^~~~~~~~~~~~~~~~~~~~~
drivers/net/ethernet/intel/igb/e1000_82575.c:2759:17:
 warning: ‘e1000_emc_temp_data’ defined but not used [-Wunused-const-variable=]
 static const u8 e1000_emc_temp_data[4] = {
                 ^~~~~~~~~~~~~~~~~~~

Move it into #ifdef block to fix this.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>