Fix build with enable_perfetto_ipc=false (e.g. Windows)

https://r.android.com/2355223 broke the build when
enable_perfetto_ipc=false. This fixes it.

Change-Id: Ib063622688f0c2f1198f2a007f107747763d2edd
(cherry picked from commit 0e2c11756bbe9a7ce5553bec8120cacb6ee2284d)
Merged-In: Ib063622688f0c2f1198f2a007f107747763d2edd
diff --git a/src/tracing/internal/system_tracing_backend_fake.cc b/src/tracing/internal/system_tracing_backend_fake.cc
index 9ca90d0..ea992a1 100644
--- a/src/tracing/internal/system_tracing_backend_fake.cc
+++ b/src/tracing/internal/system_tracing_backend_fake.cc
@@ -27,5 +27,11 @@
   return nullptr;
 }
 
+// static
+TracingBackend* SystemTracingProducerOnlyBackend::GetInstance() {
+  PERFETTO_FATAL("System tracing not implemented");
+  return nullptr;
+}
+
 }  // namespace internal
 }  // namespace perfetto