powerpc/boot: Fix random libfdt related build errors

powerpc/boot: Fix random libfdt related build errors

Once in a while I see build errors similar to the following
when building images from a clean tree.

  Building powerpc:virtex-ml507:44x/virtex5_defconfig ... failed
  ------------
  Error log:
  arch/powerpc/boot/treeboot-akebono.c:37:20: fatal error:
  	libfdt.h: No such file or directory

  Building powerpc:bamboo:smpdev:44x/bamboo_defconfig ... failed
  ------------
  Error log:
  arch/powerpc/boot/treeboot-akebono.c:37:20: fatal error:
  	libfdt.h: No such file or directory

  arch/powerpc/boot/treeboot-currituck.c:35:20: fatal error:
       libfdt.h: No such file or directory

Rebuilds will succeed.

Turns out that several source files in arch/powerpc/boot/ include
libfdt.h, but Makefile dependencies are incomplete. Let's fix that.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>