[SandboxIR][Bench] Fix missing include

In 362da64 "[SandboxIR][Bench] Test RAUW (#107440)" we started
using std::stringstream, but didn't include `<sstream>` for the
definition. This is resulting in a build failure on windows.

llvm/benchmarks/SandboxIRBench.cpp

@@ -21,6 +21,7 @@
 #include "llvm/SandboxIR/SandboxIR.h"
 #include "llvm/Support/SourceMgr.h"
 #include <memory>
+#include <sstream>
 
 using namespace llvm;