core: fix `--mix-mode` default warning

fixes the following warning that occurs after the introduction of type strictness to the plugin descriptor macros in MR !319 (merged):

WARNING : ../include/vlc_plugin.h:397: 18:  initialization of 'int64_t' {aka 'long int'} from 'void *' makes integer from pointer without a cast [-Wint-conversion]
  397 |     int64_t _v = (v); \
      |                  ^
../include/vlc_plugin.h:477:5: note: in expansion of macro 'add_int_inner'
  477 |     add_int_inner(CONFIG_ITEM_INTEGER, name, text, longtext, value)
      |     ^~~~~~~~~~~~~
libvlc-module.c:1578:5: note: in expansion of macro 'add_integer'
 1578 |     add_integer( "mix-mode", NULL, MIX_MODE_TEXT, MIX_MODE_TEXT )
      |     ^~~~~~~~~~~