igb: fix a compile warning

control these three function declarations and
definitions with same macro CONFIG_PCI_IOV

drivers/net/ethernet/intel/igb/igb_main.c:165:
warning: ‘igb_vf_configure’ declared ‘static’ but never defined
drivers/net/ethernet/intel/igb/igb_main.c:166:
warning: ‘igb_find_enabled_vfs’ declared ‘static’ but never defined
drivers/net/ethernet/intel/igb/igb_main.c:167:
warning: ‘igb_check_vf_assignment’ declared ‘static’ but never defined

Signed-off-by: RongQing Li <roy.qing.li@gmail.com>
Acked-by: Greg Rose <gregory.v.rose@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>