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

commit df73de9b

 ("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 <tushar.behera@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>