`generate-helper-types` generates invalid code when `multi-file` is not specified

[RikkiGibson]

For an rsp file like:

--config
generate-helper-types
--file
include/samplerate.h
--methodClassName
LibSampleRate
--namespace
LibSampleRateDotNet
--output
lib/file.cs
-l
samplerate.dll

Code like the following is generated. There are extra using directives etc between the helper types, so it won't compile. Valid code is generated when the multi-file option is also used.

namespace LibSampleRateDotNet
{
    public partial struct SRC_STATE_tag
    {
    }

    public unsafe partial struct SRC_DATA
    {
        [NativeTypeName("const float *")]
        public float* data_in;

        public float* data_out;

        [NativeTypeName("long")]
        public int input_frames;

        [NativeTypeName("long")]
        public int output_frames;

        [NativeTypeName("long")]
        public int input_frames_used;

        [NativeTypeName("long")]
        public int output_frames_gen;

        public int end_of_input;

        public double src_ratio;
    }
using System;
using System.Diagnostics;

namespace LibSampleRateDotNet
{
    /// <summary>Defines the type of a member as it was used in the native signature.</summary>
    [AttributeUsage(AttributeTargets.Struct | AttributeTargets.Enum | AttributeTargets.Property | AttributeTargets.Field | AttributeTargets.Parameter | AttributeTargets.ReturnValue, AllowMultiple = false, Inherited = true)]
    [Conditional("DEBUG")]
    internal sealed partial class NativeTypeNameAttribute : Attribute
    {
        private readonly string _name;

        /// <summary>Initializes a new instance of the <see cref="NativeTypeNameAttribute" /> class.</summary>
        /// <param name="name">The name of the type that was used in the native signature.</param>
        public NativeTypeNameAttribute(string name)
        {
            _name = name;
        }

        /// <summary>Gets the name of the type that was used in the native signature.</summary>
        public string Name => _name;
    }
}
using System;
using System.Diagnostics;