wasmtime-api: reserve_exact on the correct vector (#452)

Fix what looks like a copy-paste issue in wasmtime-api/src/module.rs,
which led to calling reserve_exact on one vector before pushing that
many elements into another.

wasmtime-api/src/module.rs

@@ -91,7 +91,7 @@ fn read_imports_and_exports(
  }
  SectionCode::Function => {
  let section = section.get_function_section_reader()?;
- sigs.reserve_exact(section.get_count() as usize);
+ func_sig.reserve_exact(section.get_count() as usize);
  for entry in section {
  func_sig.push(entry?);
  }