scripts/gdb: fix usage of MOD_TEXT not defined when CONFIG_MODULES=n

MOD_TEXT is only defined if CONFIG_MODULES=y which lead to loading failure
of the gdb scripts when kernel is built without CONFIG_MODULES=y:

Reading symbols from vmlinux...
Traceback (most recent call last):
  File "/foo/vmlinux-gdb.py", line 25, in <module>
    import linux.constants
  File "/foo/scripts/gdb/linux/constants.py", line 14, in <module>
    LX_MOD_TEXT = gdb.parse_and_eval("MOD_TEXT")
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
gdb.error: No symbol "MOD_TEXT" in current context.

Add a conditional check on CONFIG_MODULES to fix this error.

Link: https://lkml.kernel.org/r/20231031134848.119391-1-da.gomez@samsung.com
Fixes: b4aff751

 ("scripts/gdb: use mem instead of core_layout to get the module address")
Signed-off-by: Clément Léger <cleger@rivosinc.com>
Tested-by: Daniel Gomez <da.gomez@samsung.com>
Signed-off-by: Daniel Gomez <da.gomez@samsung.com>
Cc: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Kieran Bingham <kbingham@kernel.org>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Cc: Pankaj Raghav <p.raghav@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>