Add "-lstdc++" to c_interface_test

[hummingtree]

On Summit with CUDA 11.0.3 and GCC 9.1.0, when c_interface_test is linked against libquda_test.so it gives the following error:

libquda_test.so: undefined reference to `std::__cxx11::basic_stringstream<char, std::char_traits<char>, std::allocator<char> >::basic_stringstream()@GLIBCXX_3.4.26'
libquda_test.so: undefined reference to `std::__cxx11::basic_ostringstream<char, std::char_traits<char>, std::allocator<char> >::basic_ostringstream()@GLIBCXX_3.4.26'

Adding the "-lstdc++" flag fixes it.

[maddyscientist]

Noting we may want to revisit in this future if we are considering different STL implementations.