Fix compilation error on debian

On debian I get the following error without this patch:

Error compiling Cython file:
------------------------------------------------------------
...
        transform = ograpi.OCTNewCoordinateTransformation(src, dst)

        # Transform options.
        options = ograpi.CSLSetNameValue(
                    options, "DATELINEOFFSET", 
                    str(antimeridian_offset).encode('utf-8'))
                                                  ^
------------------------------------------------------------

fiona/_transform.pyx:127:51: Obtaining 'char *' from temporary Python value

(Cython version 0.20.1post0)