Allow `withUnsafePointer` to not mark `struct` function as `mutating`

AVCodec.swift

extension AVCodec {
  mutating func profileName(profile: Int32) -> String? {
    let result = withUnsafePointer(&self) { (codecPointer) in
      return av_get_profile_name(codecPointer, profile)
    }
    return String.fromCString(result)
  }
}

example.c

// See: https://ffmpeg.org/doxygen/2.8/libavcodec_2avcodec_8h_source.html#l03472
typedef struct AVCodec {
  // ...
} AVCodec;

// See: https://ffmpeg.org/doxygen/2.8/libavcodec_2utils_8c_source.html#l03260
const char *av_get_profile_name(const AVCodec *codec, int profile)
{
  const AVProfile *p;
  if (profile == FF_PROFILE_UNKNOWN || !codec->profiles)
    return NULL;
    
  for (p = codec->profiles; p->profile != FF_PROFILE_UNKNOWN; p++)
    if (p->profile == profile)
      return p->name;

  return NULL;
}