fix strict-prototypes warning

quantum/process_keycode/process_unicode.h

@@ -22,6 +22,7 @@ void register_hex(uint16_t hex);
 bool process_unicode(uint16_t keycode, keyrecord_t *record);
 
 #ifdef UNICODEMAP_ENABLE
+void unicode_map_input_error(void);
 bool process_unicode_map(uint16_t keycode, keyrecord_t *record);
 #endif