[msan] Fix compilation of a disabled test.

llvm-svn: 205613

compiler-rt/test/msan/dtls_test.c

@@ -54,6 +54,7 @@ int main(int argc, char *argv[]) {
 }
 #else  // BUILD_SO
 __thread long huge_thread_local_array[1 << 17];
+extern "C"
 long *GetTls() {
   return &huge_thread_local_array[0];
 }