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_coinselection.py

@@ -166,7 +166,7 @@ class CoinSelectionTracepointTest(BitcoinTestFramework):
         ctx.enable_probe(probe="coin_selection:normal_create_tx_internal", fn_name="trace_normal_create_tx")
         ctx.enable_probe(probe="coin_selection:attempting_aps_create_tx", fn_name="trace_attempt_aps")
         ctx.enable_probe(probe="coin_selection:aps_create_tx_internal", fn_name="trace_aps_create_tx")
-        self.bpf = BPF(text=coinselection_tracepoints_program, usdt_contexts=[ctx], debug=0)
+        self.bpf = BPF(text=coinselection_tracepoints_program, usdt_contexts=[ctx], debug=0, cflags=["-Wno-error=implicit-function-declaration"])
 
         self.log.info("Prepare wallets")
         self.generate(self.nodes[0], 101)