[gdb/testsuite] Fix waitpid testing in next-fork-other-thread.c

[gdb/testsuite] Fix waitpid testing in next-fork-other-thread.c

In next-fork-other-thread.c, there's this loop:
...
      do
        {
          ret = waitpid (pid, &stat, 0);
        } while (ret == EINTR);
...

The loop condition tests for "ret == EINTR" but waitpid signals EINTR by
returning -1 and setting errno to EINTR.

Fix this by changing the loop condition to "ret == -1 && errno == EINTR".

Tested on x86_64-linux.