Fix cmake compilation error in Ubuntu 18.04

[edurenye]

I had the following error in compiling it in Ubuntu 18.04 with cmake:

miraclecast/res/dispctl.vala:807.2-807.18: error: 1 missing arguments for `void GLib.Application.set_default (GLib.Application?)'
	app.set_default();

[lianzhidong]

I meet the same problem.But I annotated this line of code.It seem nothing wrong althought I didn't make it run successfully.

[edurenye]

Try with this code https://github.com/edurenye/miraclecast/tree/source-side
It's a rebase of this branch.