gdb: add missing automatic imports (#8788)

When GDB has just started, several `gdb` submodules are automatically
loaded, so user code does not have to manually import them (for instance
`import gdb.events`). Reflect that in `gdb` stubs.

stubs/gdb/gdb/__init__.pyi

@@ -10,6 +10,9 @@ from typing_extensions import TypeAlias
 
 import gdb.types
 
+# The following submodules are automatically imported
+from . import events as events, printing as printing, prompt as prompt, types as types
+
 # Basic
 
 PYTHONDIR: str