diagnostics: build fake NetworkHealthAdapter for fuzzer

The fuzzer relies on this library, but it was only being built for
tests. This change also builds it for the fuzzer.

BUG=chromium:1114475
TEST=USE="asan fuzzer" emerge-sarien diagnostics (builds)

Change-Id: I22f5b19ffb31c76b94ce32bacd0e62f8d6918581
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform2/+/2346924
Commit-Queue: Trent Begin <tbegin@chromium.org>
Commit-Queue: Paul Moy <pmoy@chromium.org>
Tested-by: Trent Begin <tbegin@chromium.org>
Auto-Submit: Trent Begin <tbegin@chromium.org>
Reviewed-by: Paul Moy <pmoy@chromium.org>
diff --git a/diagnostics/cros_healthd/network/BUILD.gn b/diagnostics/cros_healthd/network/BUILD.gn
index a56b062..1b1164f 100644
--- a/diagnostics/cros_healthd/network/BUILD.gn
+++ b/diagnostics/cros_healthd/network/BUILD.gn
@@ -31,7 +31,9 @@
     ]
     sources = [ "network_health_adapter_impl_test.cc" ]
   }
+}
 
+if (use.test || use.fuzzer) {
   source_set("libnetwork_health_adapter_test_utils") {
     configs += [ ":libnetwork_health_pkg_deps" ]
     deps = [