Fix name of Cargo feature for static linking

[cstyles]

d3879c2f29f7580da619e795b5e4883efbe5b6a8 (PR #750) added the ability to statically link to a pre-built version of librdkafka when the static-linking Cargo feature is enabled but the build script for rdkafka-sys checks for static-external instead. These need to match for static linking to take effect so let's fix the build script to use static-linking.