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>

arch/powerpc/boot/Makefile

@@ -101,7 +101,8 @@ $(addprefix $(obj)/,$(zlib-y)): \
 libfdt       := fdt.c fdt_ro.c fdt_wip.c fdt_sw.c fdt_rw.c fdt_strerror.c
 libfdtheader := fdt.h libfdt.h libfdt_internal.h
 
-$(addprefix $(obj)/,$(libfdt) libfdt-wrapper.o simpleboot.o epapr.o opal.o): \
+$(addprefix $(obj)/,$(libfdt) libfdt-wrapper.o simpleboot.o epapr.o opal.o \
+	treeboot-akebono.o treeboot-currituck.o treeboot-iss4xx.o): \
 	$(addprefix $(obj)/,$(libfdtheader))
 
 src-wlib-y := string.S crt0.S stdio.c decompress.c main.c \