libroxml: don't build docs

Don't build docs to avoid failures such as
http://autobuild.buildroot.net/results/3f1971472fe35a516b0f26ced477d33cb18280c9/

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/libroxml/libroxml.mk

@@ -30,4 +30,10 @@ define LIBROXML_CLEAN_CMDS
 	-$(MAKE) -C $(@D) clean
 endef
 
+define LIBROXML_DISABLE_DOXYGEN
+	$(SED) 's:) doxy:):' $(@D)/Makefile
+endef
+
+LIBROXML_POST_PATCH_HOOKS += LIBROXML_DISABLE_DOXYGEN
+
 $(eval $(generic-package))