builtin f32 type isn't handled

[jdm]

I'm running ctest on Rust bindings generated by bindgen, and while an older bindgen used c_float, newer versions use f32 instead. ctest happily converts c_float to float, but f32 yields errors like:

  cargo:warning=/Users/jdm/src/rust-harfbuzz/target/debug/build/harfbuzz-sys-test-256f53a27904a18b/out/all.c:2043:18: error: unknown type name 'f32'
  cargo:warning=                 f32* __test_field_type_hb_variation_t_value(struct hb_variation_t* b) {

[jdm]

Ah, I see #69.