explicitly add bindgen feature

Author: johanneskoester

hts-sys/Cargo.toml

@@ -34,6 +34,7 @@ curl = ["curl-sys", "openssl-sys"]
 gcs = ["curl"] # Google Cloud Storage support
 s3 = ["curl"] # Amazon S3 support
 static = [] # Don't dynamically link to other libraries
+bindgen = ["bindgen"]
 
 [build-dependencies]
 fs-utils = "1.1"