add pgm_read_dword for Infinity ErgoDox

avoid the following error when `UNICODEMAP_ENABLE = yes`:

```
quantum/process_keycode/process_unicodemap.c:52:21: error: implicit declaration of function 'pgm_read_dword'
```

tmk_core/common/progmem.h

@@ -7,6 +7,7 @@
 #   define PROGMEM
 #   define pgm_read_byte(p)     *((unsigned char*)p)
 #   define pgm_read_word(p)     *((uint16_t*)p)
+#   define pgm_read_dword(p)    *((uint32_t*)p)
 #endif
 
 #endif