Add fallback for the restrict keyword being unavailable to bind_gen

[doesthisusername]

Pretty much every compiler already supports restrict when compiling C, but for the ones that don't, this should improve compatibility. Additionally, using the generated livesplit_core.h in a CMake project with a minimal target feature requirement setting (c_restrict) causes IDE syntax highlighting errors when Visual Studio is used as the generator, because it lacks a c99 option. This ends up fixed too.

I didn't add a generic fallback (#define restrict ) as I doubt there's a standard way to query compiler extensions like that, especially not at gen-time, and a lot of things support __restrict. I can make it overridable for odd toolchains, if you'd like:

#if !defined(restrict) && __STDC_VERSION__ < 199901L

There theoretically could be issues with other language bindings if they're generated from livesplit_core.h, though I imagine that's unlikely, as cargo run in bind_gen seems to generate all of the languages, and I encounted no errors during that. This PR should also have identical semantics for most compilers (everything but C in a mode earlier than C99 should be the same).

P.S. I haven't made many PRs so I hope this is up to standard!