Fix: missing import in gdb pretty printers

Not sure why this is suddenly necessary, but I get an error now
that I didn't get in the past without this. The error happened when
accessing `gdb.printing.PrettyPrinter` further down.

tools/debug/gdb/blender_gdb_extension.py

@@ -27,6 +27,7 @@ __all__ = (
 )
 
 import gdb
+import gdb.printing
 from contextlib import contextmanager
 from gdb.FrameDecorator import FrameDecorator