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'):

tools/building.py

@@ -495,6 +495,9 @@ def EndBuilding(target, program = None):
         CscopeDatabase(Projects)
 
 def SrcRemove(src, remove):
+    if not src:
+        return
+
     if type(src[0]) == type('str'):
         for item in src:
             if os.path.basename(item) in remove: