decoder: fix type aliasing violation

In file included from ../../include/vlc_common.h:1190,
                 from ../../src/input/decoder_helpers.c:29:
../../src/input/decoder_helpers.c: In function ‘vlc_decoder_device_Create’:
../../include/vlc_modules.h:70:47: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
             vlc_module_load(vlc_object_logger((vlc_object_t *)(ctx)), cap, \
../../include/vlc_objects.h:64:15: note: in definition of macro ‘VLC_OBJECT’
     _Generic((x)->obj, \
               ^
../../include/vlc_modules.h:70:29: note: in expansion of macro ‘vlc_object_logger’
             vlc_module_load(vlc_object_logger((vlc_object_t *)(ctx)), cap, \
                             ^~~~~~~~~~~~~~~~~
../../src/input/decoder_helpers.c:135:20: note: in expansion of macro ‘vlc_module_load’
     priv->module = vlc_module_load(&priv->device, "decoder device", name,
                    ^~~~~~~~~~~~~~~