omcproxy: define configuration file

omcproxy's configuration is lost on every update or installation.
Avoid it by defining the configuration file.

Signed-off-by: David Santamaría Rogado <howl.nsp@gmail.com>

package/network/services/omcproxy/Makefile

@@ -8,7 +8,7 @@
 include $(TOPDIR)/rules.mk
 
 PKG_NAME:=omcproxy
-PKG_RELEASE:=8
+PKG_RELEASE:=9
 
 PKG_SOURCE_PROTO:=git
 PKG_SOURCE_URL=$(PROJECT_GIT)/project/omcproxy.git
@@ -28,6 +28,10 @@ define Package/omcproxy
   TITLE:=IGMPv3 and MLDv2 Multicast Proxy
 endef
 
+define Package/omcproxy/conffiles
+/etc/config/omcproxy
+endef
+
 CMAKE_OPTIONS += -DWITH_LIBUBOX=1
 
 define Package/omcproxy/install