test: fix usdt undeclared function errors on mantis

Recently usage of undeclared functions became an error rather than a
warning, in C2x. https://reviews.llvm.org/D122983?id=420290

This change has migrated into the build tools of Ubuntu 23.10 which now
causes the USDT tests to fail to compile, see
https://github.com/bitcoin/bitcoin/issues/28600

Fix this by setting `-Wno-error=implicit-function-declaration` for the
tracing programs.

test/functional/interface_usdt_validation.py

@@ -94,7 +94,7 @@ class ValidationTracepointTest(BitcoinTestFramework):
         ctx.enable_probe(probe="validation:block_connected",
                          fn_name="trace_block_connected")
         bpf = BPF(text=validation_blockconnected_program,
-                  usdt_contexts=[ctx], debug=0)
+                  usdt_contexts=[ctx], debug=0, cflags=["-Wno-error=implicit-function-declaration"])
 
         def handle_blockconnected(_, data, __):
             event = ctypes.cast(data, ctypes.POINTER(Block)).contents