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>

drivers/net/ethernet/intel/igb/igb_main.c

@@ -173,7 +173,9 @@ static int igb_check_vf_assignment(struct igb_adapter *adapter);
 #endif
 
 #ifdef CONFIG_PM
+#ifdef CONFIG_PM_SLEEP
 static int igb_suspend(struct device *);
+#endif
 static int igb_resume(struct device *);
 #ifdef CONFIG_PM_RUNTIME
 static int igb_runtime_suspend(struct device *dev);
@@ -6709,6 +6711,7 @@ static int __igb_shutdown(struct pci_dev *pdev, bool *enable_wake,
 }
 
 #ifdef CONFIG_PM
+#ifdef CONFIG_PM_SLEEP
 static int igb_suspend(struct device *dev)
 {
 	int retval;
@@ -6728,6 +6731,7 @@ static int igb_suspend(struct device *dev)
 
 	return 0;
 }
+#endif /* CONFIG_PM_SLEEP */
 
 static int igb_resume(struct device *dev)
 {