ecore-drm: fix ecore_drm_devices_get() to return a const value

this is the internal device list, we're pretty hosed if someone decides
to free it because there's no const

src/lib/ecore_drm/Ecore_Drm.h

@@ -746,6 +746,6 @@ EAPI void ecore_drm_device_pointer_xy_get(Ecore_Drm_Device *dev, int *x, int *y)
  * @ingroup Ecore_Drm_Device_Group
  * @since 1.14
  */
-EAPI Eina_List *ecore_drm_devices_get(void);
+EAPI const Eina_List *ecore_drm_devices_get(void);
 
 #endif

src/lib/ecore_drm/ecore_drm_device.c

@@ -353,7 +353,7 @@ ecore_drm_device_close(Ecore_Drm_Device *dev)
    return EINA_TRUE;
 }
 
-EAPI Eina_List *
+EAPI const Eina_List *
 ecore_drm_devices_get(void)
 {
    return drm_devices;