Add gtest as a dependency for neteq_quality_test_support.

Was removed in Patch Set 5 of https://codereview.webrtc.org/2252413002
but shouldn't have been, since it's actually required.

https://cs.chromium.org/chromium/src/third_party/webrtc/modules/audio_coding/neteq/tools/neteq_quality_test.h?l=17

BUG=webrtc:6228
NOTRY=True

Review-Url: https://codereview.webrtc.org/2262173003
Cr-Commit-Position: refs/heads/master@{#13851}

webrtc/modules/audio_coding/BUILD.gn

@@ -1181,6 +1181,7 @@ if (rtc_include_tests) {
     deps = [
       ":neteq",
       ":neteq_unittest_tools",
+      "//testing/gtest",
       "//third_party/gflags",
     ]
   }