[PATCH] drm/panel: make drm_panel.h self-contained

[Jani Nikula]

Fix build warning if drm_panel.h is built with CONFIG_OF=n or
CONFIG_DRM_PANEL=n and included without the prerequisite err.h:

./include/drm/drm_panel.h: In function ‘of_drm_find_panel’:
./include/drm/drm_panel.h:203:9: error: implicit declaration of function ‘ERR_PTR’ [-Werror=implicit-function-declaration]
  return ERR_PTR(-ENODEV);
         ^~~~~~~
./include/drm/drm_panel.h:203:9: error: returning ‘int’ from a function with return type ‘struct drm_panel *’ makes pointer from integer without a cast [-Werror=int-conversion]
  return ERR_PTR(-ENODEV);
         ^~~~~~~~~~~~~~~~

Fixes: 5fa8e4a22182 ("drm/panel: Make of_drm_find_panel() return an ERR_PTR() instead of NULL")
Cc: Boris Brezillon <boris.brezillon at bootlin.com>
Cc: Thierry Reding <treding at nvidia.com>
Signed-off-by: Jani Nikula <jani.nikula at intel.com>

---

I don't know if there's a combo where this actually fails, so I'm not
adding cc: stable. It's just something I hit when playing with other
code.
---
 include/drm/drm_panel.h | 1 +
 1 file changed, 1 insertion(+)

diff --git a/include/drm/drm_panel.h b/include/drm/drm_panel.h
index 8c738c0e6e9f..26377836141c 100644
--- a/include/drm/drm_panel.h
+++ b/include/drm/drm_panel.h
@@ -24,6 +24,7 @@
 #ifndef __DRM_PANEL_H__
 #define __DRM_PANEL_H__
 
+#include <linux/err.h>
 #include <linux/errno.h>
 #include <linux/list.h>
 
-- 
2.20.1