doc/Makefile: fix deps

The "$(SRC): $(FOOTER)" dependency listed is incorrect, as criu.txt file
is not dependent on footer.txt. Having said that, criu.8 in fact do
depend on footer.txt but that was not reflected in Makefile. Fix it.

While at it, add the custom.xsl dependency, which was also missing.

Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

Documentation/Makefile

@@ -38,10 +38,7 @@ $(FOOTER): ../Makefile.versions
 	$(Q) echo ":man version:   $(CRIU_VERSION)"	>> $@
 	$(Q) echo ":man manual:    CRIU Manual"		>> $@
 
-$(SRC): $(FOOTER)
-	@true
-
-%.8: %.txt
+%.8: %.txt $(FOOTER) custom.xsl
 	$(call msg-gen, $@)
 	$(Q) $(ASCIIDOC) -b docbook -d manpage -o $(patsubst %.8,%.xml,$@) $<
 	$(Q) $(XMLTO) man -m custom.xsl $(patsubst %.8,%.xml,$@) 2>/dev/null