Unbreak build with libc++

[jbeich]

Regressed by #58

$ export CC=clang CXX=clang++ CXXFLAGS=-stdlib=libc++
$ cmake -G Ninja -B _build
$ cmake --build _build
[...]
libhyprcursor/hyprcursor.cpp:23:27: error: implicit instantiation of undefined template 'std::basic_stringstream<char>'
   23 |         std::stringstream envXdgStream(envXdgData);
      |                           ^
/usr/include/c++/v1/__fwd/sstream.h:29:28: note: template is declared here
   29 | class _LIBCPP_TEMPLATE_VIS basic_stringstream;
      |                            ^