tools: fix SrcRemove on empty src

If the src is empty list, it will crash at:

    IndexError: list index out of range:
      File "/home/xxx/src/SConstruct", line 39:
        objs = PrepareBuilding(env, RTT_ROOT, has_libcpu=True)
      ...
      File "/home/xxx/src/drivers/SConscript", line 12:
        SrcRemove(src, src_need_remove)
      File "/home/rt-thread-stable/tools/building.py", line 496:
        if type(src[0]) == type('str'):

Commit d33df46f1921f241199e7520e384e851840571ef in master.
Merge pull request #352 from grissiom/fix-tools-srcremove-st

tools: fix SrcRemove on empty src