build: fix doc generation errors in parallel builds

The $(dir) function used to construct OBJDIRS includes a trailing slash
in the names returned, which GNU make 3.82 does not match to the
slash-less 'doc' in the documentation dependencies, causing parallel
build to fail.  Adding a slash fixes this and still works with make
3.81.

Signed-off-by: Mans Rullgard <mans@mansr.com>

doc/Makefile

@@ -40,7 +40,7 @@ doc/%.1: TAG = MAN
 doc/%.1: doc/%.pod $(GENTEXI)
 	$(M)pod2man --section=1 --center=" " --release=" " $< > $@
 
-$(DOCS): | doc
+$(DOCS): | doc/
 
 install-progs-$(CONFIG_DOC): install-man