binderVendorDoubleLoadTest: stability assert

As these tests/binaries are being imported to other
build systems over time, add one canary check.

Bug: N/A
Test: N/A
Change-Id: I86be1ee659694e301aa8b3bf4a2dffaefaee9720

Author: Steven Moreland

libs/binder/ndk/tests/binderVendorDoubleLoadTest.cpp

@@ -30,6 +30,8 @@
 #include <gtest/gtest.h>
 #include <sys/prctl.h>
 
+static_assert(FLAG_PRIVATE_LOCAL != 0, "Build system configuration breaks stability");
+
 using namespace android;
 using ::android::binder::Status;
 using ::android::internal::Stability;