programs: test: add C++ specific commands to cpp_dummy_build

Add C++ specific instructions to the generated *.cpp source file so
that the build will fail in case a C compiler is used.

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>

programs/test/generate_cpp_dummy_build.sh

@@ -73,8 +73,12 @@ EOF
 
     cat <<'EOF'
 
+#include <iostream>
+
 int main()
 {
+    std::cout << "CPP dummy build\n";
+
     mbedtls_platform_context *ctx = NULL;
     mbedtls_platform_setup(ctx);
     mbedtls_printf("CPP Build test passed\n");