fix: add missing standard library includes in xinterpreter.cpp

- Add <cstring> for std::strlen
- Add <sstream> for std::istringstream
- Add <string> for std::getline

Resolves include-cleaner warnings by explicitly including headers
for all used standard library symbols. This improves code maintainability
and prevents reliance on indirect includes.

Author: AhmedFatthy1040

src/xinterpreter.cpp

@@ -15,6 +15,10 @@
 #include "xeus-cpp/xinterpreter.hpp"
 #include "xeus-cpp/xmagics.hpp"
 
+#include <cstring>    // for std::strlen
+#include <sstream>    // for std::istringstream
+#include <string>     // for std::getline
+
 #include "xinput.hpp"
 #include "xinspect.hpp"
 #ifndef EMSCRIPTEN