[compiler-rt][rtsan] Fix fseek test build (unused variable warning). (#123116)

Author: devnexen

compiler-rt/lib/rtsan/tests/rtsan_test_interceptors_posix.cpp

@@ -545,6 +545,7 @@ TEST_F(RtsanOpenedFileTest, FtelloDieWhenRealtime) {
 
 TEST_F(RtsanOpenedFileTest, RewindDieWhenRealtime) {
   int end = fseek(GetOpenFile(), 0, SEEK_END);
+  EXPECT_THAT(end, Eq(0));
   auto Func = [this]() { rewind(GetOpenFile()); };
 
   ExpectRealtimeDeath(Func, "rewind");