kbuild: silence documentation GEN xml messages according to $(quiet)

Add rules for gen_xml and its quiet & silent variants.  This causes "make -s"
to be silent for gen_xml.
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

Documentation/DocBook/Makefile

@@ -187,8 +187,11 @@ quiet_cmd_fig2png = FIG2PNG $@
 
 ###
 # Rule to convert a .c file to inline XML documentation
+       gen_xml = :
+ quiet_gen_xml = echo '  GEN     $@'
+silent_gen_xml = :
 %.xml: %.c
-	@echo '  GEN     $@'
+	@$($(quiet)gen_xml)
 	@(                            \
 	   echo "<programlisting>";   \
 	   expand --tabs=8 < $< |     \