mfd: syscon: Include linux/err.h to fix build error

commit df73de9b0d412 ("mfd: syscon: Return -ENOSYS if CONFIG_MFD_SYSCON
is not enabled") introduced fallbacks for APIs, but missed out on adding
the header file. This would work only if linux/err.h is also included
in the source code from where this file is included. It would be better
to include linux/err.h in file to remove possible build errors.

Without this patch, we get following and similar build errors if this
header file is included in some source file and CONFIG_MFD_SYSCON is
not enabled.

include/linux/mfd/syscon.h: In function ‘syscon_node_to_regmap’:
include/linux/mfd/syscon.h:30:2: error: implicit declaration of function ‘ERR_PTR’ [-Werror=implicit-function-declaration]
return ERR_PTR(-ENOSYS);
^
include/linux/mfd/syscon.h:30:18: error: ‘ENOSYS’ undeclared (first use in this function)
return ERR_PTR(-ENOSYS);
^
Signed-off-by: Tushar Behera <[e-mail]>
Signed-off-by: Lee Jones <[e-mail]>

include/linux/mfd/syscon.h

···
 #ifndef __LINUX_MFD_SYSCON_H__
 #define __LINUX_MFD_SYSCON_H__
 
+#include <linux/err.h>
+
 struct device_node;
 
 #ifdef CONFIG_MFD_SYSCON