[RFC] build: build with -Wmissing-declarations/prototypes #36743

The header declared AddInputs(..., bool rbf), which was never defined,
while the definition takes std::optional<bool> and has no declaration.

These were declared in fa6b061fc1 but never defined or used.

The definitions were removed in 43c1f5b8d7, but the declarations were
left behind.

Match the definition, and the m_idle_since member.

These functions are only used within the file that defines them, and
have no declaration elsewhere, so -Wmissing-prototypes (clang) and
-Wmissing-declarations (GCC) warn about them. Give them internal
linkage.

fail_test_if_executed() is [[maybe_unused]], because TRACEPOINT discards
its arguments when tracing is disabled.

LLVMFuzzerTestOneInput and LLVMFuzzerInitialize are called by libFuzzer,
so they can't be static. Declare them first, to avoid
-Wmissing-prototypes / -Wmissing-declarations warnings.

This is GCC's equivalent of clang's -Wmissing-prototypes for C++.

It was declared locally in guiutil.cpp, so the definition was never
checked against the declaration.