add_c_file: tolerate ./ in filenames.

scripts/maint/add_c_file.py

@@ -216,6 +216,9 @@ def run(fn):
     add them to include.am.
     """
 
+    if fn.startswith("./"):
+        fn = fn[2:]
+
     cf = makeext(fn, "c")
     hf = makeext(fn, "h")