reallocarray: eliminate compilation warnings (#3798)

reallocarray: eliminate compilation warnings (#3798)

there are compile warnings below for klockstat.

```bash
  CC       klockstat.o
klockstat.c: In function ‘print_stats’:
klockstat.c:396:12: warning: implicit declaration of function ‘reallocarray’; did you mean ‘realloc’? [-Wimplicit-function-declaration]
    stats = reallocarray(stats, stats_sz, sizeof(void *));
            ^~~~~~~~~~~~
            realloc
klockstat.c:396:10: warning: assignment to ‘struct stack_stat **’ from ‘int’ makes pointer from integer without a cast [-Wint-conversion]
    stats = reallocarray(stats, stats_sz, sizeof(void *));
          ^
  BINARY   klockstat
```

Defining _GNU_SOURCE fixed the problem.