build: Do not explicitly add the doc directory to the OBJDIRS list.

Now that a documentation generator is built in the doc directory,
this is no longer necessary.  Fixes the Make warning:
Makefile:188: target `doc' given more than once in the same rule.

doc/Makefile

@@ -41,7 +41,6 @@ doc/%.1: doc/%.pod $(GENTEXI)
 	$(M)pod2man --section=1 --center=" " --release=" " $< > $@
 
 $(DOCS): | doc
-OBJDIRS += doc
 
 install-progs-$(CONFIG_DOC): install-man