Fix method signature of platform_utils.FileDescriptorStreams._create_stream()

Change-Id: Ib80e4ec5e540d97488e7564703ddbcb74350fdfd
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/251836
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Rostislav Krasny <rostigm@gmail.com>

platform_utils.py

@@ -80,7 +80,7 @@ class FileDescriptorStreams(object):
     """
     raise NotImplementedError
 
-  def _create_stream(fd, dest, std_name):
+  def _create_stream(self, fd, dest, std_name):
     """ Creates a new stream wrapping an existing file descriptor.
     """
     raise NotImplementedError