Add library.json

Add library.json for VS Code PlatformIO

{
  "name": "speex",
  "version": "1.2.1",
  "headers": "speex.h",
  "build": {
    "srcDir": "./libspeex"
  },
  "homepage": "https://www.speex.org/",
  "repository":
  {
      "type": "git",
      "url": "https://gitlab.xiph.org/xiph/speex.git"
  }
}