pinctrl: sh-pfc: Add missing #include <linux/errno.h>

Source files using -Exxx error codes should include <linux/errno.h>.
On ARM, this header file is included indirectly; on SuperH, it is not,
leading to "error: ‘EINVAL’ undeclared" failures when enabling
compile-testing later.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>