Fix up include in cppx example
Fix up include in cppx example

dkm/gcc-explorer,dkm/gcc-explorer,mattgodbolt/compiler-explorer,dkm/gcc-explorer,dkm/gcc-explorer,dkm/gcc-explorer,mattgodbolt/gcc-explorer,dkm/gcc-explorer,mattgodbolt/compiler-explorer,mattgodbolt/compiler-explorer,dkm/gcc-explorer,mattgodbolt/compiler-explorer,mattgodbolt/compiler-explorer,dkm/gcc-explorer,mattgodbolt/gcc-explorer,dkm/gcc-explorer,mattgodbolt/gcc-explorer,mattgodbolt/gcc-explorer,mattgodbolt/compiler-explorer,mattgodbolt/compiler-explorer,mattgodbolt/compiler-explorer,mattgodbolt/gcc-explorer