SCons: List .gen.cpp sources explicitly to avoid globbing errors

Whenever we change the name (or remove) generated cpp files with the `.gen.cpp`
extension, users run into build issues when switching between branches (i.e.
switching before and after the name change/removal). This is because we glob
`*.cpp` so if a now-obsolete file from a previous build is present, we'll
include it too, potentially leading to bugs or compilation failure (due to
missing headers or invalid code).

So globbing patterns in `add_source_files` will now skip files ending with
`.gen.cpp`, which should instead be passed explicitly where they're used.

editor/SCsub

@@ -101,6 +101,7 @@ if env["tools"]:
     )
 
     env.add_source_files(env.editor_sources, "*.cpp")
+    env.add_source_files(env.editor_sources, "register_exporters.gen.cpp")
 
     SConscript("debugger/SCsub")
     SConscript("fileserver/SCsub")