powerpc: Fix build dependencies for epapr.c which needs libfdt.h

Currently, the build can (very rarely) fail to build because libfdt.h has
not been created or is in the process of being copied.

Signed-off-by: Matthew McClintock <msm@freescale.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>