[libcxx][pretty printers] Import gdb module in gdb feature check

Earlier versions of GDB do not do this automatically.
(from my checks 8.3 does not and 9.2 does)

libcxx/utils/libcxx/test/features.py

@@ -171,6 +171,7 @@ def check_gdb(cfg):
   # Using the quit command here means that gdb itself exits, not just
   # the "python <...>" command.
   test_src = """\
+import gdb
 try:
   gdb.Breakpoint(\"main\").commands=\"foo\"
 except AttributeError: