gdb/hurd: remove VLA usage

Compilation will fail with -Werror=vla, which seems to be the default.

Note that we don't need to allocate num_threads + 1 since the matching
algorithm works only on the num_threads as returned by task_threads.

Change-Id: I276928d0ff3c52c7c7fe4edb857e5789cdabfcf7