UBUNTU: SAUCE: amd: Include errno.h

BugLink: http://bugs.launchpad.net/bugs/1546572

Fixes compile failure on non-amd64 arches:

drivers/gpu/drm/amd/amdgpu/../powerplay/hwmgr/tonga_thermal.c: In function 'tonga_fan_ctrl_get_fan_speed_percent':
drivers/gpu/drm/amd/amdgpu/../powerplay/hwmgr/tonga_thermal.c:80:11: error: 'EINVAL' undeclared (first use in this function)
   return -EINVAL;
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>

drivers/gpu/drm/amd/powerplay/inc/hwmgr.h

@@ -23,6 +23,7 @@
 #ifndef _HWMGR_H_
 #define _HWMGR_H_
 
+#include <linux/errno.h>
 #include "amd_powerplay.h"
 #include "pp_instance.h"
 #include "hardwaremanager.h"