Fix compile warning for abort() when MRB_GC_STRESS is defined

My clang  warn as below.

```
CC    src/gc.c -> build/host/src/gc.o
/Users/koji/work/mruby/mruby/src/gc.c:385:5: warning: implicitly declaring
      library function 'abort' with type 'void (void)
      __attribute__((noreturn))'
    abort();
    ^
/Users/koji/work/mruby/mruby/src/gc.c:385:5: note: please include the header
      <stdlib.h> or explicitly provide a declaration for 'abort'
1 warning generated.

```