scripts/gdb: support !CONFIG_MODULES gracefully

If CONFIG_MODULES is not enabled, lx-lsmod tries to find a non-existent
symbol and generates an unfriendly traceback:

  (gdb) lx-lsmod
  Address    Module                  Size  Used by
  Traceback (most recent call last):
    File "scripts/gdb/linux/modules.py", line 75, in invoke
      for module in module_list():
    File "scripts/gdb/linux/modules.py", line 24, in module_list
      module_ptr_type = module_type.get_type().pointer()
    File "scripts/gdb/linux/utils.py", line 28, in get_type
      self._type = gdb.lookup_type(self._name)
  gdb.error: No struct type named module.
  Error occurred in Python command: No struct type named module.

Catch the error and return an empty module_list() for a clean command
output as follows:

  (gdb) lx-lsmod
  Address    Module                  Size  Used by
  (gdb)

Link: http://lkml.kernel.org/r/94d533819437408b85ae5864f939dd7ca6fbfcd6.1462865983.git.jan.kiszka@siemens.com


Signed-off-by: Kieran Bingham <kieran.bingham@linaro.org>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>