getdeps: stop error with build --clean and --src-dir=.

Summary:
X-link: facebookincubator/zstrong#1026

fix the error shown with build --clean and --src-dir=.

Reviewed By: jdelliot

Differential Revision: D64985193

fbshipit-source-id: f933adb45a7385b13965b89e421b4f9284cf0ecb

Author: ahornby

build/fbcode_builder/getdeps/fetcher.py

@@ -155,6 +155,9 @@ def hash(self) -> str:
     def get_src_dir(self):
         return self.path
 
+    def clean(self) -> None:
+        pass
+
 
 class SystemPackageFetcher(object):
     def __init__(self, build_options, packages) -> None: