Add "rt" to libs of GN build linux sandbox unittests.

This is causing a compile failure in component mode.

TBR=dpranke

Review URL: https://codereview.chromium.org/566023003

Cr-Commit-Position: refs/heads/master@{#294658}

sandbox/linux/BUILD.gn

@@ -86,6 +86,8 @@ test("sandbox_linux_unittests") {
     "//testing/gtest",
   ]
 
+  libs = [ "rt" ]
+
   if (compile_suid_client) {
     sources += [
       "suid/client/setuid_sandbox_client_unittest.cc",