[gdb/python] Fix cpychecker error in recpy_bt_goto

When using cpychecker, we run into this error:
...
gdb/python/py-record-btrace.c: \
  In function ‘PyObject* recpy_bt_goto(PyObject*, PyObject*)’:
gdb/python/py-record-btrace.c:783:25: error: Mismatching type in call to \
  PyArg_ParseTuple with format code "O" [-Werror]
   if (!PyArg_ParseTuple (args, "O", &obj))
  argument 3 ("&obj") had type
    "const struct recpy_element_object * *"
  but was expecting
    "struct PyObject * *"
  for format code "O"
...

Fix this by using a new variable of the expected type instead.

Build and reg-tested on x86_64-linux.

2018-10-05  Tom de Vries  <tdevries@suse.de>

	* python/py-record-btrace.c (recpy_bt_goto): Fix type mismatch in
	PyArg_ParseTuple call.