Define HGVERSION to silence IDE warning

[kjkent]

Hi there! I hope the unsolicited PR is okay; please close if not.

When opening the file in vscode, clangd highlights the following "error" in the code as it doesn't gather that HGVERSION is inserted in Makefile:

Expected ')'clang(expected)
	mdns-repeater.c(339, 9): To match this '('

This PR fixes that, if it's something you feel is worth adding.