This reverts commit 45384ef.

Extract a simple function function to a simple header with no circular
dependencies.

Next we'll use bindgen to generate Rust bindings for this function.

Ticket: OISF#7341

Add a build.rs to generate Rust bindings to specific C functions at
build time using the bindgen crate. As can be seen in build.rs, we
currently only pull in the test header, "app-layer-ext.h" and only
output items starting with "SC".

Bindgen generates the bindings in a file named "bindings.rs" in the
target/ direction, which "sys.rs" will statically "include" at
compiling name, making these bindings available under package
"crate::sys".

"build.rs" had to be placed in the non-standard location of
"src/build.rs" (its usually alongside Cargo.toml) to satisfy the
out-of-tree build requirements of distcheck.

Note that bindgen is also available as a command line tool which could
be used instead of integrating this at compile time, however the tool
requires a newer version of Rust than our MSRV, and may present
additional issues with respect to autoconf and distcheck.

This function is now named SCAppLayerStateGetProgressFn, note how its
definition differs from ours:

pub type SCAppLayerStateGetProgressFn = ::std::option::Option<
    unsafe extern "C" fn(
        alstate: *mut ::std::os::raw::c_void,
        direction: u8,
    ) -> ::std::os::raw::c_int,
>;

Our previous definition:

pub type StateGetProgressFn = unsafe extern "C" fn (*mut c_void, u8) -> c_int;

The main differing being wrapped in an option which makes sense as its
a function pointer that could be NULL/None, but does require wrapping
it in an Option now.

Rust will now fail, as the discrepancy between i8 and i32 is picked up.