gdb, testsuite, python: Add missing imports.

Removing the pretty printer (bound_registers.py) in the next commit
leads to failures due to a missing import of 'gdb.printing':

"AttributeError: module 'gdb' has no attribute 'printing'".

Add this import to each file requiring it, as it's not imported by the
pretty-printer anymore.

Approved-By: Andrew Burgess <aburgess@redhat.com>

gdb/python/lib/gdb/dap/varref.py

@@ -18,6 +18,7 @@ from collections import defaultdict
 from contextlib import contextmanager
 
 import gdb
+import gdb.printing
 
 from .server import client_bool_capability
 from .startup import DAPException, in_gdb_thread

gdb/testsuite/gdb.python/py-pp-cast.py

@@ -13,6 +13,7 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
+import gdb.printing
 
 class PpIntPrinter(object):
     def __init__(self, val):

gdb/testsuite/gdb.python/py-pp-re-notag.py

@@ -16,7 +16,7 @@
 from time import asctime, gmtime
 
 import gdb  # silence pyflakes
-
+import gdb.printing
 
 class TimePrinter:
     def __init__(self, val):

gdb/testsuite/gdb.python/py-prettyprint-stub.py

@@ -16,6 +16,7 @@
 # This file is part of the GDB testsuite.
 # It tests Python-based pretty-printing of stubs.
 
+import gdb.printing
 
 class SPrinter:
     def __init__(self, val):