Fix main() signature to comply with libc++ testing requirements

Changed main() signature from main() to main(int, char**) as per the Testing libc++ documentation requirement (https://libcxx.llvm.org/TestingLibcxx.html).

The documentation states: "Some platforms where libc++ is tested have requirement on the signature of main and require main to explicitly return a value. Therefore the typical main function should look like: int main(int, char**)"

Author: harsha08-2k6

test_copy_file_multithreaded.cpp

@@ -78,7 +78,7 @@ void test_copy_file_multithreaded() {
     fs::remove_all(dest_dir);
 }
 
-int main() {
+int main(int, char**) {
     std::cout << "Testing copy_file in multithreaded environment (Issue #169261)\n";
     test_copy_file_multithreaded();
     std::cout << "Test finished\n";