include Directory.Build.targets file in solution

Author: jjw24

Flow.Launcher.sln

@@ -53,6 +53,7 @@ Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "Solution Items", "Solution
 		.gitattributes = .gitattributes
 		.gitignore = .gitignore
 		appveyor.yml = appveyor.yml
+		Directory.Build.targets = Directory.Build.targets
 		Scripts\flowlauncher.nuspec = Scripts\flowlauncher.nuspec
 		LICENSE = LICENSE
 		Scripts\post_build.ps1 = Scripts\post_build.ps1