Keymap: mitosis/datagrok: bug fix (#3854)

I don't know how this slipped past the test suite in the last merge, but
this syntax I tried is invalid and doesn't compile. Fixed.

keyboards/mitosis/keymaps/datagrok/keymap.c

@@ -3,7 +3,7 @@
 #ifdef AUDIO_ENABLE
 #include "audio.h"
 #ifdef DEFAULT_LAYER_SONGS
-extern float default_layer_songs[][][];
+extern float default_layer_songs[][16][2];
 #endif
 #endif