Add gn support for libsrtp

BUILD.gn

 # Copyright (c) 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # This is the root build file for GN. GN will start processing by loading this
 # file, and recursively load all dependencies until all dependencies are either
 # resolved or known not to exist (which will cause the build to fail). So if
 # you add a new build file, there must be some path of dependencies from this
 # file to your new one or GN won't know about it.
 
@@ skipping 41 matching lines @@
     "//third_party/angle:translator",
     "//third_party/brotli",
     "//third_party/ffmpeg",
     "//third_party/flac",
     "//third_party/harfbuzz-ng",
     "//third_party/hunspell",
     "//third_party/iccjpeg",
     "//third_party/icu",
     "//third_party/leveldatabase",
     "//third_party/libpng",
+    "//third_party/libsrtp",
     "//third_party/libusb",
     "//third_party/libwebm",
     "//third_party/libwebp",
     "//third_party/libxslt",
     "//third_party/libyuv",
     "//third_party/lzma_sdk",
     "//third_party/mesa",
     "//third_party/ots",
     "//third_party/protobuf:protobuf_lite",
     "//third_party/qcms",
@@ skipping 89 matching lines @@
     ]
   }
 
   if (use_ozone) {
     deps += [
       "//ui/events/ozone:events_ozone",
       "//ui/events/ozone:events_ozone_evdev",
     ]
   }
 }