simpleperf: fix presubmit test.

cuttlefish is changed to mount tracefs in
/sys/kernel/tracing instead of /sys/kernel/debug/tracing.
So try both tracefs locations.

Bug: 148521916
Test: run simpleperf_unit_test.
Change-Id: I6052fd3feb14d5e93334e5e95704e1fa173fac25
diff --git a/simpleperf/nonlinux_support/nonlinux_support.cpp b/simpleperf/nonlinux_support/nonlinux_support.cpp
index 1726b58..deb3b1d 100644
--- a/simpleperf/nonlinux_support/nonlinux_support.cpp
+++ b/simpleperf/nonlinux_support/nonlinux_support.cpp
@@ -39,6 +39,10 @@
   return true;
 }
 
+const char* GetTraceFsDir() {
+  return nullptr;
+}
+
 namespace simpleperf {
 
 class DummyOfflineUnwinder : public OfflineUnwinder {