NvOptimusEnablement build error

Just tried to build with today's git head of imgui/implot/implot_demos with gcc 10.3.0 and see:
```
[ 92%] Building CXX object CMakeFiles/app.dir/common/Fonts/RobotoItalic.cpp.o
/usr/src/local/implot_demos/common/App.cpp: In constructor ‘App::App(std::string, int, int, int, const char**)’:
/usr/src/local/implot_demos/common/App.cpp:152:5: error: ‘NvOptimusEnablement’ was not declared in this scope
152 | NvOptimusEnablement = AmdPowerXpressRequestHighPerformance = result["gpu"].as();
| ^~~~~~~~~~~~~~~~~~~
/usr/src/local/implot_demos/common/App.cpp:152:27: error: ‘AmdPowerXpressRequestHighPerformance’ was not declared in this scope
152 | NvOptimusEnablement = AmdPowerXpressRequestHighPerformance = result["gpu"].as();
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```