kbuild/ppc: Fix build of zlib in arch/ppc/boot/lib

$(addprefix ...) needs a directory relative to current directory, because
kbuild prefixes the filename with '$(obj)/'
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

arch/ppc/boot/lib/Makefile

@@ -4,7 +4,7 @@
 
 CFLAGS_kbd.o	+= -Idrivers/char
 
-lib-y := $(addprefix lib/zlib_inflate/,infblock.o infcodes.o inffast.o \
-				       inflate.o inftrees.o infutil.o)
+lib-y := $(addprefix ../../../../lib/zlib_inflate/, \
+           infblock.o infcodes.o inffast.o inflate.o inftrees.o infutil.o)
 lib-y += div64.o
 lib-$(CONFIG_VGA_CONSOLE) += vreset.o kbd.o