clean up the warnings when GC_PROFILE is set

it would be a good practice to printf `size_t` with `%zu`

```
CC    src/gc.c -> build/host/src/gc.o
/home/liyazhou/code/mruby/src/gc.c: In function ‘mrb_incremental_gc’:
/home/liyazhou/code/mruby/src/gc.c:999:3: warning: format ‘%d’ expects argument of type ‘int’, but argument 3 has type ‘size_t’ [-Wformat]
/home/liyazhou/code/mruby/src/gc.c:999:3: warning: format ‘%d’ expects argument of type ‘int’, but argument 3 has type ‘size_t’ [-Wformat]
/home/liyazhou/code/mruby/src/gc.c:999:3: warning: format ‘%d’ expects argument of type ‘int’, but argument 3 has type ‘size_t’ [-Wformat]
/home/liyazhou/code/mruby/src/gc.c: In function ‘mrb_full_gc’:
/home/liyazhou/code/mruby/src/gc.c:1028:3: warning: format ‘%d’ expects argument of type ‘int’, but argument 3 has type ‘size_t’ [-Wformat]
/home/liyazhou/code/mruby/src/gc.c:1028:3: warning: format ‘%d’ expects argument of type ‘int’, but argument 3 has type ‘size_t’ [-Wformat]
/home/liyazhou/code/mruby/src/gc.c:1028:3: warning: format ‘%d’ expects argument of type ‘int’, but argument 3 has type ‘size_t’ [-Wformat]
```