post/Makefile: make sure to use the correct flags

ARFLAGS was not set, which caused "ppc_8xx-ar: creating libgenpost.a"
messages to be printed.

Signed-off-by: Wolfgang Denk <wd@denx.de>

post/Makefile

@@ -21,6 +21,7 @@
 # MA 02111-1307 USA
 #
 
+include $(TOPDIR)/config.mk
 include $(TOPDIR)/include/autoconf.mk
 
 LIB				= libpost.a