[clang][examples] Fix build failure (NFC)

/llvm-project/clang/examples/LLVMPrintFunctionNames/LLVMPrintFunctionNames.cpp:36:20:
error: cannot use incomplete type 'llvm::Module' as a range
    for (auto &F : M)
                   ^

clang/examples/LLVMPrintFunctionNames/LLVMPrintFunctionNames.cpp

@@ -18,6 +18,7 @@
 #include "clang/Frontend/CompilerInstance.h"
 #include "clang/Frontend/FrontendPluginRegistry.h"
 #include "clang/Sema/Sema.h"
+#include "llvm/IR/Module.h"
 #include "llvm/IR/PassManager.h"
 #include "llvm/Passes/OptimizationLevel.h"
 #include "llvm/Passes/PassBuilder.h"