expose feature for gdal with bindgen

This is necessary for linking against gdal versions whose bindings
aren't yet included in gdal-sys, which is common when using a newly
released gdal.

Also removed redundant gdal-sys dependency - gdal already includes that,
and geozero doesn't use it directly.

geozero/Cargo.toml

@@ -15,7 +15,8 @@ license.workspace = true
 default = ["with-svg", "with-wkt", "with-geo", "with-geojson"]
 with-arrow = ["dep:arrow2"]
 with-csv = ["dep:csv", "with-wkt"]
-with-gdal = ["dep:gdal", "dep:gdal-sys"]
+with-gdal = ["dep:gdal"]
+with-gdal-bindgen = [ "with-gdal", "gdal?/bindgen" ]
 with-geo = ["dep:geo-types"]
 with-geojson = ["dep:geojson"]
 with-geos = ["dep:geos"]