Add librt to default libs on linux

third_party/widevine/cdm/BUILD.gn

 # Copyright 2014 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.
 
 import("//build/config/chrome_build.gni")
 import("//build/config/features.gni")
 import("//chrome/version.gni")  # TODO layering violation
 import("//media/cdm/ppapi/ppapi_cdm_adapter.gni")
 import("//third_party/widevine/cdm/widevine.gni")
 
@@ skipping 116 matching lines @@
     deps = [
       ":version_h",
       ":widevinecdm",
       ":widevinecdmadapter_resources",
       "//ppapi/cpp",
     ]
 
     if (is_linux) {
       ldflags =
           [ rebase_path("$root_out_dir/libwidevinecdm.so", root_build_dir) ]
-      libs = [ "rt" ]
     } else if (is_win) {
       ldflags =
           [ rebase_path("$root_out_dir/widevinecdm.dll.lib", root_build_dir) ]
     } else if (is_mac) {
       ldflags =
           [ rebase_path("$root_out_dir/libwidevinecdm.dylib", root_build_dir) ]
     }
   }
 } else {
   # Placeholder when we're not compiling the adapter.
   group("widevinecdmadapter") {
   }
 }
 
 # This target exists for tests to depend on that pulls in a runtime dependency
 # on the license server.
 source_set("widevine_test_license_server") {
   if (is_chrome_branded && is_linux) {
     deps = [
       # TODO(jrummell)
       # This target should be removed and targets should have data_deps on this target:
       #"//third_party/widevine/test/license_server/license_server.gyp:test_license_server"
     ]
   }
 }