Cargo: configure cargo-c to use vendored .h

Presently we pre-generate the rustls.h header file using `cbindgen`,
commit the result to `src/rustls.h`, and check that the generated result
matches the checked-in result in CI.

The new experimental cargo-c build regenerates its own header file using
`cbindgen` by default unless told to do otherwise. We'd prefer it didn't
do this because we're using a `cbindgen` feature that requires nightly
rust.

This commit updates the `Cargo.toml` capi metadata to tell cargo-c to
skip generation of its own header file. We then configure the
pre-generated checked-in header file as an asset to be copied into the
install include directory.

This better matches how the `Makefile` build allowed building the static
lib without needing nightly rust or `cbindgen`.

Cargo.toml

@@ -41,6 +41,7 @@ regex = "1.9.6"
 [package.metadata.capi.header]
 name = "rustls"
 subdirectory = false
+generation = false # Prefer a vendored .h
 
 [package.metadata.capi.library]
 name = "rustls"
@@ -50,3 +51,6 @@ rustflags = "-Cmetadata=rustls-ffi"
 [package.metadata.capi.pkg_config]
 name = "rustls"
 filename = "rustls"
+
+[package.metadata.capi.install.include]
+asset = [{from = "src/rustls.h", to = "" }]