test: Add imports for util bpf_cflags

This is required for the next commit.

test/functional/interface_usdt_coinselection.py

@@ -1,5 +1,5 @@
 #!/usr/bin/env python3
-# Copyright (c) 2022 The Bitcoin Core developers
+# Copyright (c) 2022-present The Bitcoin Core developers
 # Distributed under the MIT software license, see the accompanying
 # file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
@@ -17,6 +17,7 @@ from test_framework.util import (
     assert_equal,
     assert_greater_than,
     assert_raises_rpc_error,
+    bpf_cflags,
 )
 
 coinselection_tracepoints_program = """