igb: fix warning about unused function

This patch fixes a warning about unused function when CONFIG_PM_SLEEP
is not selected in the kernel config:

igb_main.c: warning: `igb_suspend` defined but not used [W-unused-function]

Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>