luci-compat: add transitional package

Add an empty luci-compat package to allow forward compatibility with
openwrt-19.07 and master for packages that support all three branches.

Ref: https://forum.openwrt.org/t/luci-compat-and-18-06/48369
Signed-off-by: Jo-Philipp Wich <jo@mein.io>

modules/luci-compat/Makefile

@@ -0,0 +1,19 @@
+#
+# Copyright (C) 2019 Jo-Philipp Wich <jo@mein.io>
+#
+# This is free software, licensed under the Apache License, Version 2.0 .
+#
+
+include $(TOPDIR)/rules.mk
+
+PKG_NAME:=luci-compat
+
+LUCI_TYPE:=mod
+LUCI_BASENAME:=compat
+
+LUCI_TITLE:=LuCI compatibility libraries (transitional package)
+LUCI_DEPENDS:=+luci-base
+
+include ../../luci.mk
+
+# call BuildPackage - OpenWrt buildroot signature