davfs2: add secrets file to conffiles definition

davfs2 username and password information is typically stored in
/etc/davfs2/secrets. This information should be kept across sysupgrades.

Signed-off-by: Matthew Hagan <mnhagan88@gmail.com>

net/davfs2/Makefile

@@ -54,6 +54,7 @@ endef
 
 define Package/davfs2/conffiles
 /etc/davfs2/davfs2.conf
+/etc/davfs2/secrets
 endef
 
 CONFIGURE_ARGS += \