extres: regulator: fix the build without FDT

The regnode interface is currently only built for FDT, thus not
applicable to ACPI-only kernels. Move the "regnode_if.h" include
underneath a previously existing FDT cpp segment.

Reviewed by: andrew, imp, manu

(cherry picked from commit 23c318ed86c4b4e6c474823a944f5fb9f2f1c2fd)