[Tsan] Do not declare std_suppressions when not used

[kutuzov.viktor.84]

Summary

Otherwise, the build fail with the "variable defined, but not used" kind of warning on FreeBSD.

[dvyukov]

LGTM

lib/tsan/rtl/tsan_suppressions.cc

 #include "sanitizer_common/sanitizer_placement_new.h"
 #include "sanitizer_common/sanitizer_suppressions.h"
 #include "tsan_suppressions.h"
 #include "tsan_rtl.h"
 #include "tsan_flags.h"
 #include "tsan_mman.h"
 #include "tsan_platform.h"
 
+#ifndef SANITIZER_GO
 // Suppressions for true/false positives in standard libraries.
 static const char *const std_suppressions =
 // Libstdc++ 4.4 has data races in std::string.
 // See http://crbug.com/181502 for an example.
 "race:^_M_rep$\n"
 "race:^_M_is_leaked$\n"
 // False positive when using std <thread>.
 // Happens because we miss atomic synchronization in libstdc++.
 // See http://llvm.org/bugs/show_bug.cgi?id=17066 for details.
 "race:std::_Sp_counted_ptr_inplace<std::thread::_Impl\n";
 
 // Can be overriden in frontend.
-#ifndef SANITIZER_GO
 extern "C" const char *WEAK __tsan_default_suppressions() {
   return 0;
 }
 #endif
 
 namespace __tsan {
 
 ALIGNED(64) static char suppression_placeholder[sizeof(SuppressionContext)];