Update Unity.gitignore

Ignore the build directory in case a Unity build is initiated in the project.

Unity.gitignore

@@ -1,6 +1,7 @@
 [Ll]ibrary/
 [Tt]emp/
 [Oo]bj/
+[Bb]uild/
 
 # Autogenerated VS/MD solution and project files
 /*.csproj