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.

(cherry picked from commit c133480531)

editor/SCsub

@@ -91,6 +91,7 @@ if env["tools"]:
     env.CommandNoCache("#editor/builtin_fonts.gen.h", flist, run_in_subprocess(editor_builders.make_fonts_header))
 
     env.add_source_files(env.editor_sources, "*.cpp")
+    env.add_source_files(env.editor_sources, "register_exporters.gen.cpp")
 
     SConscript("collada/SCsub")
     SConscript("doc/SCsub")