Fix: Compilation error when using CTFType::String{,NoWrite}

[skoudmar]

Previously the rust type was std::ffi::CStr which is similar to str and does not have known size at compile-time and cannot be used as a function argument. Use reference &CStr instead.

Tracepoint definition

provider
        .create_class("class")
        .add_field("c_string", CTFType::String)
        .instantiate("tracepoint");

The compile error

   Compiling lttng-test v0.1.0
error[E0277]: the size for values of type `[i8]` cannot be known at compilation time
   --> REDACTED/out/tracepoints.rs:8:46
    |
8   |         pub(in super::super) fn tracepoint<>(a0: ::std::ffi::CStr) {
    |                                              ^^ doesn't have a size known at compile-time
    |
    = help: within `CStr`, the trait `Sized` is not implemented for `[i8]`, which is required by `CStr: Sized`
note: required because it appears within the type `CStr`
   --> HOME/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ffi/c_str.rs:108:12
    |
108 | pub struct CStr {
    |            ^^^^
help: function arguments must have a statically known size, borrowed types always have a known size
    |
8   |         pub(in super::super) fn tracepoint<>(a0: &::std::ffi::CStr) {
    |                                                  +

error[E0308]: mismatched types
  --> REDACTED/out/tracepoints.rs:10:61
   |
10 |                 super::detail::provider_class_tracepoint_tp(a0)
   |                 ------------------------------------------- ^^ expected `*const i8`, found `CStr`
   |                 |
   |                 arguments to this function are incorrect
   |
   = note: expected raw pointer `*const i8`
                   found struct `CStr`
note: function defined here
  --> REDACTED/out/lttng-tracepoints/tracepoint_library_link_name/tracepoints.rs:4:12
   |
4  |     pub fn provider_class_tracepoint_tp(c_string_arg: *const ::std::os::raw::c_char);
   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Some errors have detailed explanations: E0277, E0308.
For more information about an error, try `rustc --explain E0277`.
error: could not compile `lttng-test` (bin "lttng-test") due to 2 previous errors