[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.

gdb/testsuite/gdb.threads/next-fork-other-thread.c

@@ -44,7 +44,7 @@ forker (void *arg)
       do
 	{
 	  ret = waitpid (pid, &stat, 0);
-	} while (ret == EINTR);
+	} while (ret == -1 && errno == EINTR);
 
       assert (ret == pid);
       assert (WIFEXITED (stat));